s390/debug: replace zero-length array with flexible-array member

There are numerous patches which convert zero-length arrays with a
flexible-array member. Convert the remaining s390 occurrences.
Suggested-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://github.com/KSPP/linux/issues/78
Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.htmlSigned-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/debug.c

@@ -60,7 +60,7 @@ typedef struct {
 	 * except of floats, and long long (32 bit)
 	 *
 	 */
-	long args[0];
+	long args[];
 } debug_sprintf_entry_t;
 
 /* internal function prototyes */