x86/vdso: Include vdso/processor.h

__vdso_getcpu is declared in a header but this is not included
before the definition, causing a W=1 warning:

arch/x86/entry/vdso/vgetcpu.c:13:1: error: no previous prototype for '__vdso_getcpu' [-Werror=missing-prototypes]
arch/x86/entry/vdso/vdso32/../vgetcpu.c:13:1: error: no previous prototype for '__vdso_getcpu' [-Werror=missing-prototypes]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://lore.kernel.org/all/20230516193549.544673-17-arnd%40kernel.org