protobuf: Add licensing info and update copyright

Signed-off-by: Guillaume Déflache <guillaume.deflache@ibwag.com>

libs/protobuf/Makefile

@@ -1,5 +1,5 @@
 #
-# Copyright (C) 2007 OpenWrt.org
+# Copyright (C) 2007-2015 OpenWrt.org
 #
 # This is free software, licensed under the GNU General Public License v2.
 # See /LICENSE for more information.
@@ -15,6 +15,9 @@ PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.bz2
 PKG_SOURCE_URL:=http://protobuf.googlecode.com/files
 PKG_MD5SUM:=a72001a9067a4c2c4e0e836d0f92ece4
 
+PKG_LICENSE:=BSD-3-Clause
+PKG_LICENSE_FILES:=LICENSE
+
 PKG_BUILD_DEPENDS:=protobuf/host
 PKG_USE_MIPS16:=0