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/fcntl/fcntl16.c b/testcases/kernel/syscalls/fcntl/fcntl16.c
index 93f7c89..44b6a80 100644
--- a/testcases/kernel/syscalls/fcntl/fcntl16.c
+++ b/testcases/kernel/syscalls/fcntl/fcntl16.c
@@ -651,7 +651,7 @@
 {
 
 	int lc;
-	char *msg;
+	const char *msg;
 
 	if ((msg = parse_opts(ac, av, NULL, NULL)) != NULL) {
 		tst_brkm(TBROK, NULL, "OPTION PARSING ERROR - %s", msg);