build: Add missing "static" keyword where it should be used

Otherwise we fail to build with -Werror=missing-prototypes.

https://bugzilla.gnome.org/show_bug.cgi?id=687385

gio/tests/gdbus-peer-object-manager.c

@@ -84,7 +84,7 @@ mock_interface_get_info (GDBusInterfaceSkeleton *skeleton)
   return &interface_info;
 }
 
-GVariant *
+static GVariant *
 mock_interface_get_property (GDBusConnection *connection,
                              const gchar *sender,
                              const gchar *object_path,

gio/tests/gdbus-peer.c

@@ -1682,7 +1682,7 @@ codegen_service_thread_func (gpointer user_data)
 }
 
 
-gboolean
+static gboolean
 codegen_quit_mainloop_timeout (gpointer data)
 {
   g_main_loop_quit (loop);

gio/tests/resources.c

@@ -487,7 +487,7 @@ test_uri_query_info (void)
   g_resource_unref (resource);
 }
 
-void
+static void
 test_uri_file (void)
 {
   GResource *resource;