protobuf: add license information

Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/protobuf/protobuf.mk

@@ -6,6 +6,8 @@
 PROTOBUF_VERSION = 2.4.1
 PROTOBUF_SOURCE = protobuf-$(PROTOBUF_VERSION).tar.gz
 PROTOBUF_SITE = http://protobuf.googlecode.com/files/
+PROTOBUF_LICENSE = BSD-3c
+PROTOBUF_LICENSE_FILES = COPYING.txt
 
 # N.B. Need to use host protoc during cross compilation.
 PROTOBUF_DEPENDENCIES = host-protobuf