opentracker: add PKG_LICENSE information

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

net/opentracker/Makefile

@@ -12,6 +12,7 @@ PKG_VERSION:=20130804
 PKG_RELEASE:=1
 PKG_REV:=954f5029dfa17734dc408336ef710c192268e8a4
 PKG_MAINTAINER:=Daniel Golle <daniel@makrotopia.org>
+PKG_LICENSE:=Beerware
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.bz2
 PKG_SOURCE_URL:=git://erdgeist.org/opentracker