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/fcntl15.c b/testcases/kernel/syscalls/fcntl/fcntl15.c
index 39152e4..7853d8d 100644
--- a/testcases/kernel/syscalls/fcntl/fcntl15.c
+++ b/testcases/kernel/syscalls/fcntl/fcntl15.c
@@ -543,7 +543,7 @@
 int main(int ac, char **av)
 {
 	int lc;
-	char *msg;
+	const char *msg;
 
 	int fail = 0;