protobuf: fix legal-info

The file changed from COPYING.txt to LICENSE, fixes:
http://autobuild.buildroot.net/results/c40/c40e9b6030584a4fd8b3e46f78f6d0702d1dc11e/

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/protobuf/protobuf.mk

@@ -7,7 +7,7 @@
 PROTOBUF_VERSION = v2.6.1
 PROTOBUF_SITE = $(call github,google,protobuf,$(PROTOBUF_VERSION))
 PROTOBUF_LICENSE = BSD-3c
-PROTOBUF_LICENSE_FILES = COPYING.txt
+PROTOBUF_LICENSE_FILES = LICENSE
 # no configure script
 PROTOBUF_AUTORECONF = YES