x86: fix pgtable_t build breakage

Commit 2f569afd

 ("CONFIG_HIGHPTE vs.
sub-page page tables") caused some build breakage due to pgtable_t only
getting declared in the CONFIG_X86_PAE case.

Move the declaration outside the PAE section.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>