x86/vdso: Fix -Wmissing-prototypes warnings

Fix those:

In file included from arch/x86/entry/vdso/vdso32/vclock_gettime.c:4:
arch/x86/entry/vdso/vdso32/../vclock_gettime.c:70:5: warning: no previous prototype for ‘__vdso_clock_gettime64’ [-Wmissing-prototypes]
70 | int __vdso_clock_gettime64(clockid_t clock, struct __kernel_timespec *ts)
|

In file included from arch/x86/entry/vdso/vdso32/vgetcpu.c:3:
arch/x86/entry/vdso/vdso32/../vgetcpu.c:13:1: warning: no previous prototype for ‘__vdso_getcpu’ [-Wmissing-prototypes]
13 | __vdso_getcpu(unsigned *cpu, unsigned *node, struct getcpu_cache *unused)
| ^~~~~~~~~~~~~

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/202302070742.iYcnoJwk-lkp@intel.com

x86/vdso: Enable x86 to use common headers

Enable x86 to use only the common headers in the implementation
of the vDSO library.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200320145351.32292-24-vincenzo.frascino@arm.com