utils/osmo-config-merge: explicitly return from main()

This change fixes the following compiler warning:

  osmo-config-merge.c: In function ‘main’:
  osmo-config-merge.c:268:1: warning: control reaches end
                             of non-void function [-Wreturn-type]

Despite it isn't critical, let's get rid of this warning.

Change-Id: I7a80a85c8b3180dc086cd9fd20356aab16ea8100
diff --git a/utils/osmo-config-merge.c b/utils/osmo-config-merge.c
index 3762c67..c76e42d 100644
--- a/utils/osmo-config-merge.c
+++ b/utils/osmo-config-merge.c
@@ -265,4 +265,6 @@
 	/* make AddressSanitizer / LeakSanitizer happy by recursively freeing the trees */
 	talloc_free(patch_tree);
 	talloc_free(base_tree);
+
+	return 0;
 }