s390/mm: silence compiler warning when compiling without CONFIG_PGSTE

If CONFIG_PGSTE is not set (e.g. when compiling without KVM), GCC complains:

  CC      arch/s390/mm/pgtable.o
arch/s390/mm/pgtable.c:413:15: warning: ‘pmd_alloc_map’ defined but not
 used [-Wunused-function]
 static pmd_t *pmd_alloc_map(struct mm_struct *mm, unsigned long addr)
               ^~~~~~~~~~~~~

Wrap the function with "#ifdef CONFIG_PGSTE" to silence the warning.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>