s390/mm: fix no previous prototype warnings in maccess.c

Fix -Wmissing-prototypes warnings caused by missing maccess.h include.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 2f0e8aae ("s390/mm: rework memcpy_real() to avoid DAT-off mode")
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>