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/fdatasync/fdatasync02.c b/testcases/kernel/syscalls/fdatasync/fdatasync02.c
index ec0b0e6..3cf2572 100644
--- a/testcases/kernel/syscalls/fdatasync/fdatasync02.c
+++ b/testcases/kernel/syscalls/fdatasync/fdatasync02.c
@@ -110,7 +110,7 @@
 int main(int argc, char **argv)
 {
 	int lc;
-	char *msg;
+	const char *msg;
 
 	if ((msg = parse_opts(argc, argv, NULL, NULL)) != NULL) {
 		tst_brkm(TBROK, NULL, "OPTION PARSING ERROR - %s", msg);