um: Fix -Wmissing-prototypes warnings for __vdso_*

The VDSO functions are defined as globals and intended to be called
from userspace. Let's just workaround the -Wmissing-prototypes warnings
by declaring them locally.

This will address below -Wmissing-prototypes warnings:

arch/x86/um/vdso/um_vdso.c:16:5: warning: no previous prototype for ‘__vdso_clock_gettime’ [-Wmissing-prototypes]
arch/x86/um/vdso/um_vdso.c:30:5: warning: no previous prototype for ‘__vdso_gettimeofday’ [-Wmissing-prototypes]
arch/x86/um/vdso/um_vdso.c:44:21: warning: no previous prototype for ‘__vdso_time’ [-Wmissing-prototypes]
arch/x86/um/vdso/um_vdso.c:57:1: warning: no previous prototype for ‘__vdso_getcpu’ [-Wmissing-prototypes]

While at it, also fix the "WARNING: Prefer 'unsigned int *' to bare
use of 'unsigned *'" checkpatch warning.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Signed-off-by: Richard Weinberger <richard@nod.at>