s390/mm: Fix ERROR: "__node_distance" undefined!

Fixes:
ERROR: "__node_distance" [drivers/nvme/host/nvme-core.ko] undefined!
make[1]: *** [scripts/Makefile.modpost:92: __modpost] Error 1
make: *** [Makefile:1275: modules] Error 2
+ exit 1

Signed-off-by: Justin M. Forbes <[e-mail]>
Signed-off-by: Heiko Carstens <[e-mail]>
Signed-off-by: Martin Schwidefsky <[e-mail]>

arch/s390/numa/numa.c

···
 {
 	return mode->distance ? mode->distance(a, b) : 0;
 }
+EXPORT_SYMBOL(__node_distance);
 
 int numa_debug_enabled;