um: include linux/stddef.h for __always_inline

When compiling against musl, their shipped <stddef.h> doesn't have
__always_inline. So instead explicitly include the kernel uapi header,
<linux/stddef.h>, which does.

This prevents the following build error:

In file included from arch/x86/um/shared/sysdep/stub.h:11,
                 from arch/um/kernel/skas/clone.c:14:
arch/x86/um/shared/sysdep/stub_64.h:111:23: error: expected ‘;’ before ‘void’
  111 | static __always_inline void *get_stub_page(void)
      |                       ^~~~~
      |                       ;
make[4]: *** [scripts/Makefile.build:249: arch/um/kernel/skas/clone.o] Error 1

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Richard Weinberger <richard@nod.at>