Fix build with GNU make 3.81

Makefile

@@ -106,12 +106,13 @@ refresh_feeds: FORCE
 	)
 
 
-export define FEEDS
+define FEEDS
 src-link gluon ../../packages_gluon
 src-link packages ../../packages_openwrt
 src-link routing ../../packages_routing
 src-svn luci http://svn.luci.subsignal.org/luci/tags/0.11.1/contrib/package
 endef
+export define
 
 feeds: FORCE
 	rm -f feeds.conf