Include <fcntl.h> instead of <sys/fcntl.h>

[marv]

Fixes the following warnings when building on systems using the musl libc:

In file included from sysfs.c:4:
/usr/riscv64-unknown-linux-musl/include/sys/fcntl.h:1:2: warning: redirecting incorrect #include <sys/fcntl.h> to <fcntl.h> [-W#warnings]
    1 | #warning redirecting incorrect #include <sys/fcntl.h> to <fcntl.h>
      |  ^
1 warning generated.

On systems using glibc this is also just a wrapper:

$ cat /usr/include/sys/fcntl.h

[andikleen]

Thanks!