[glib] build: don't forget to clean generated portal files

[Cosimo Cecchi <cosimoc src gnome org>]

commit f4e2047f2010417e8323cb5bb4fa843957fdba45
Author: Cosimo Cecchi <cosimo endlessm com>
Date:   Mon Jul 11 15:13:56 2016 -0700

    build: don't forget to clean generated portal files

 gio/Makefile.am |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/gio/Makefile.am b/gio/Makefile.am
index a98c0df..4ef98aa 100644
--- a/gio/Makefile.am
+++ b/gio/Makefile.am
@@ -341,6 +341,7 @@ endif
 
 xdp_dbus_built_sources = xdp-dbus.c xdp-dbus.h
 BUILT_SOURCES += $(xdp_dbus_built_sources)
+CLEANFILES += $(xdp_dbus_built_sources)
 
 portal_interfaces = \
        org.freedesktop.portal.NetworkMonitor.xml \