Clean up warnings from core build

[nderjung]

The current build is not "clean" and has some outstanding warnings, including:

/tmp/build/80754af9/built/unikraft/lib/nolibc/stdio.c: In function 'fputc':
/tmp/build/80754af9/built/unikraft/lib/nolibc/stdio.c:497:5: warning: 'ret' may be used uninitialized in this function [-Wmaybe-uninitialized]
  497 |  if (ret == 1)
      |     ^
/tmp/build/80754af9/built/unikraft/lib/vfscore/mount.c: In function '__uk_syscall_r_sync':
/tmp/build/80754af9/built/unikraft/lib/vfscore/mount.c:380:1: warning: no return statement in function returning non-void [-Wreturn-type]
  380 | }
      | ^
/tmp/build/80754af9/built/unikraft/lib/vfscore/main.c: In function 'ioctl':
/tmp/build/80754af9/built/unikraft/lib/vfscore/main.c:686:41: warning: passing argument 3 of 'uk_syscall_e_ioctl' makes integer from pointer without a cast [-Wint-conversion]
  686 |  return uk_syscall_e_ioctl(fd, request, arg);
      |                                         ^~~
      |                                         |
      |                                         void *
In file included from /tmp/build/80754af9/built/unikraft/lib/vfscore/main.c:54:
/tmp/build/80754af9/built/unikraft/lib/vfscore/main.c:649:10: note: expected 'long int' but argument is of type 'void *'
  649 |   void*, arg)
/tmp/build/80754af9/built/unikraft/lib/syscall_shim/include/uk/syscall.h:108:41: note: in definition of macro 'UK_S_ARG_LONG'
  108 | #define UK_S_ARG_LONG(type, arg)   long arg
      |                                         ^~~
/tmp/build/80754af9/built/unikraft/lib/syscall_shim/include/uk/syscall.h:100:54: note: in expansion of macro 'UK_ARG_MAP1'
  100 | #define UK_ARG_MAP2(m, type, arg, ...) m(type, arg), UK_ARG_MAP1(m, __VA_ARGS__)
      |                                                      ^~~~~~~~~~~
/tmp/build/80754af9/built/unikraft/lib/syscall_shim/include/uk/syscall.h:101:54: note: in expansion of macro 'UK_ARG_MAP2'
  101 | #define UK_ARG_MAP3(m, type, arg, ...) m(type, arg), UK_ARG_MAP2(m, __VA_ARGS__)
      |                                                      ^~~~~~~~~~~
/tmp/build/80754af9/built/unikraft/include/uk/essentials.h:158:29: note: in expansion of macro 'UK_ARG_MAP3'
  158 | #define __UK_CONCAT_X(a, b) a##b
      |                             ^
/tmp/build/80754af9/built/unikraft/lib/syscall_shim/include/uk/syscall.h:199:13: note: in expansion of macro 'UK_ARG_MAPx'
  199 |  long ename(UK_ARG_MAPx(x, UK_S_ARG_LONG, __VA_ARGS__))  \
      |             ^~~~~~~~~~~
/tmp/build/80754af9/built/unikraft/lib/syscall_shim/include/uk/syscall.h:219:37: note: in expansion of macro '__UK_LLSYSCALL_R_DEFINE'
  219 | #define _UK_LLSYSCALL_R_DEFINE(...) __UK_LLSYSCALL_R_DEFINE(__VA_ARGS__)
      |                                     ^~~~~~~~~~~~~~~~~~~~~~~
/tmp/build/80754af9/built/unikraft/lib/syscall_shim/include/uk/syscall.h:221:2: note: in expansion of macro '_UK_LLSYSCALL_R_DEFINE'
  221 |  _UK_LLSYSCALL_R_DEFINE(__UK_SYSCALL_DEF_NARGS(__VA_ARGS__), \
      |  ^~~~~~~~~~~~~~~~~~~~~~
/tmp/build/80754af9/built/unikraft/lib/vfscore/main.c:648:1: note: in expansion of macro 'UK_LLSYSCALL_R_DEFINE'
  648 | UK_LLSYSCALL_R_DEFINE(int, ioctl, int, fd, unsigned long int, request,
      | ^~~~~~~~~~~~~~~~~~~~~