[S390] add missing __tlb_flush_global() for !CONFIG_SMP

Fix this compiler error for !CONFIG_SMP:

  CC      arch/s390/mm/pgtable.o
arch/s390/mm/pgtable.c: In function ‘gmap_flush_tlb’:
arch/s390/mm/pgtable.c:202:3: error: implicit declaration of function ‘__tlb_flush_global’ [-Werror=implicit-function-declaration]
cc1: some warnings being treated as errors

Signed-off-by: Jan Glauber <jang@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>