s390/vmlogrdr: Use array instead of string initializer

Compiling vmlogrdr with GCC 15 generates this warning:

  CC [M]  drivers/s390/char/vmlogrdr.o
drivers/s390/char/vmlogrdr.c:126:29: error: initializer-string for array
 of ‘char’ is too long [-Werror=unterminated-string-initialization]
  126 |         { .system_service = "*LOGREC ",

Given that the system_service array intentionally contains a non-null
terminated string use an array initializer, instead of string
initializer to get rid of this warning.

Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>