x86/numa/32: Include missing <asm/pgtable_areas.h>

The __vmalloc_start_set declaration is in a header that is not included
in numa_32.c in current linux-next:

  arch/x86/mm/numa_32.c: In function 'initmem_init':
  arch/x86/mm/numa_32.c:57:9: error: '__vmalloc_start_set' undeclared (first use in this function)
     57 |         __vmalloc_start_set = true;
        |         ^~~~~~~~~~~~~~~~~~~
  arch/x86/mm/numa_32.c:57:9: note: each undeclared identifier is reported only once for each function it appears in

Add an explicit #include.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240403202344.3463169-1-arnd@kernel.org