testcases: Constify char *msg variable.

The parse_opts() now returns const char * instead of char * this commit
fixes all warnings caused by the change.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
diff --git a/testcases/kernel/syscalls/signalfd4/signalfd4_02.c b/testcases/kernel/syscalls/signalfd4/signalfd4_02.c
index de40d71..807c3d9 100644
--- a/testcases/kernel/syscalls/signalfd4/signalfd4_02.c
+++ b/testcases/kernel/syscalls/signalfd4/signalfd4_02.c
@@ -125,7 +125,7 @@
 	sigset_t ss;
 	int fd, fl;
 	int lc;
-	char *msg;
+	const char *msg;
 
 	msg = parse_opts(argc, argv, NULL, NULL);
 	if (msg != NULL) {