testsuite: Fix up memchr-3.c test [PR108991]

The newly added dg-warning directive was missing the comment argument,
so the target selector was treated as comment and the warning was expected
on all targets when it should be expected only on llp64 targets.

2023-03-03  Jakub Jelinek  <jakub@redhat.com>

	PR testsuite/108991
	* gcc.dg/memchr-3.c: Add missing comment argument to dg-warning
	before target selector.

gcc/testsuite/gcc.dg/memchr-3.c

@@ -6,7 +6,7 @@
 typedef __INT8_TYPE__  int8_t;
 typedef __INT32_TYPE__ int32_t;
 
-extern void* memchr (const void*, int, long); /* { dg-warning "-Wbuiltin-declaration-mismatch" { target llp64 } } */
+extern void* memchr (const void*, int, long); /* { dg-warning "-Wbuiltin-declaration-mismatch" "" { target llp64 } } */
 
 struct SX
 {