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/mem/oom/oom02.c b/testcases/kernel/mem/oom/oom02.c
index 1fd199b..3f17400 100644
--- a/testcases/kernel/mem/oom/oom02.c
+++ b/testcases/kernel/mem/oom/oom02.c
@@ -48,7 +48,7 @@
 
 int main(int argc, char *argv[])
 {
-	char *msg;
+	const char *msg;
 	int lc;
 
 	msg = parse_opts(argc, argv, NULL, NULL);