powerpc/32: Fix compile error caused by pud_t/pgt_t confusion

PPC32 is still using asm-generic/4level-fixup.h, but asm-powerpc/page.h
was defining pud_t and pgd_t.  Depending on the order in which files
got included, this could result in a compilation error.  Tweak the ifdef
so that page.h doesn't try to define pud_t on ppc32 (which uses 2-level
page tables).

Signed-off-by: Paul Mackerras <paulus@samba.org>