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>

arch/s390/include/asm/uaccess.h

@@ -47,8 +47,6 @@ raw_copy_to_user(void __user *to, const void *from, unsigned long n);
 int __put_user_bad(void) __attribute__((noreturn));
 int __get_user_bad(void) __attribute__((noreturn));
 
-#ifdef CONFIG_HAVE_MARCH_Z10_FEATURES
-
 union oac {
 	unsigned int val;
 	struct {
@@ -71,6 +69,8 @@ union oac {
 	};
 };
 
+#ifdef CONFIG_HAVE_MARCH_Z10_FEATURES
+
 #define __put_get_user_asm(to, from, size, oac_spec)			\
 ({									\
 	int __rc;							\