Fix prototype mismatch of strlen() in libc.h

[visitorckw]

Commit f95a700 ("Correct return type of strlen function") changed the return type of strlen() from int32_t to size_t in its definition, but did not update the prototype in include/lib/libc.h accordingly. This causes a build failure due to a conflicting type declaration:

  CC    build/lib/libc.o
lib/libc.c:23:8: error: conflicting types for 'strlen'; have 'size_t(const char *)' {aka 'unsigned int(const char *)'}
   23 | size_t strlen(const char *s)
      |        ^~~~~~
In file included from lib/libc.c:1:
./include/lib/libc.h:73:9: note: previous declaration of 'strlen' with type 'int32_t(const char *)' {aka 'int(const char *)'}
   73 | int32_t strlen(const char *s1);
      |         ^~~~~~
make: *** [Makefile:53: build/lib/libc.o] Error 1

Update the strlen() prototype in the header file to match the new return type and fix the build error.

Fixes: f95a700 ("Correct return type of strlen function")