fix compiler warnings: return 0 in main(), in 3 tests

Change-Id: I9e49ceba6b0a8dffd331e5707667d9bb007f3ec7
diff --git a/tests/gtp/gtpie_test.c b/tests/gtp/gtpie_test.c
index fe2d502..8d87dcc 100644
--- a/tests/gtp/gtpie_test.c
+++ b/tests/gtp/gtpie_test.c
@@ -124,4 +124,5 @@
 	/* TODO: gtpie_encaps() */
 	/* TODO: gtpie_encaps2() */
 	/* TODO: gtpie_getie(), gtpie_exist(), gtpie_get*() */
+	return 0;
 }