make C=2 ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- xxx.o

When I use the command above to do a 'make C=2' check on any object file,
the following warnings are always output:

  CHECK   arch/arm64/kernel/vdso/vgettimeofday.c
arch/arm64/kernel/vdso/vgettimeofday.c:9:5: warning:
 symbol '__kernel_clock_gettime' was not declared. Should it be static?
arch/arm64/kernel/vdso/vgettimeofday.c:15:5: warning:
 symbol '__kernel_gettimeofday' was not declared. Should it be static?
arch/arm64/kernel/vdso/vgettimeofday.c:21:5: warning:
 symbol '__kernel_clock_getres' was not declared. Should it be static?

Therefore, the declaration of the three functions is added to eliminate
these common warnings to provide a clean output.
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/r/20230713115831.777-1-thunder.leizhen@huawei.comSigned-off-by: Will Deacon <will@kernel.org>