mtr: Add PKG_LICENSE information

Signed-off-by: Jonathan McCrohan <jmccrohan@gmail.com>

net/mtr/Makefile

@@ -14,6 +14,8 @@ PKG_RELEASE:=1
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
 PKG_SOURCE_URL:=ftp://ftp.bitwizard.nl/mtr
 PKG_MD5SUM:=5e665c617e5659b6ec3e201ee7488eb1
+PKG_LICENSE:=GPL-2.0+
+PKG_LICENSE_FILES:=COPYING
 
 PKG_INSTALL:=1