x86: vdso/vma.c declare vdso_enabled and arch_setup_additional_pages before they get used

Impact: cleanup, address sparse warnings

Addresses the problem pointed out by these sparse warning:
  arch/x86/vdso/vma.c:19:28: warning: symbol 'vdso_enabled' was not declared. Should it be static?
  arch/x86/vdso/vma.c:101:5: warning: symbol 'arch_setup_additional_pages' was not declared. Should it be static?

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
LKML-Reference: <1239548845.4170.2.camel@localhost.localdomain>
Signed-off-by: Ingo Molnar <mingo@elte.hu>