lib: arm/arm64: Add missing include for alloc_page.h in pgtable.h

pgtable.h is used only by mmu.c, where it is included after alloc_page.h.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>