s390/uaccess: fix compile error

Compiling with e.g MARCH=z900 results in compile errors:

   arch/s390/lib/uaccess.c: In function 'copy_from_user_mvcos':
>> arch/s390/lib/uaccess.c:65:15: error: variable 'spec' has initializer but incomplete type
      65 |         union oac spec = {

Therefore make definition of union oac visible for all MARCHs.

Reported-by: kernel test robot <lkp@intel.com>
Cc: Nico Boehr <nrb@linux.ibm.com>
Cc: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
Fixes: 012a224e ("s390/uaccess: introduce bit field for OAC specifier")
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>