Fix build error in vma.c

This fixes the following much-reported build issue:

arch/x86/entry/vdso/vma.c: In function ‘map_vdso’:
arch/x86/entry/vdso/vma.c:175:9: error:
        implicit declaration of function ‘pvclock_pvti_cpu0_va’

on some arches and configurations.

Thanks to Guenter for being persistent enough to get it fixed :)

Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>