python-protobuf: fix legal info

It uses the same tarball as protobuf hence suffers the same problem as
commit f700463c. Fixes:
http://autobuild.buildroot.net/results/428/4282281e5582b0da8ed3ff44f4d6d86f716ea333/

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

package/python-protobuf/python-protobuf.mk

@@ -8,7 +8,7 @@ PYTHON_PROTOBUF_VERSION = $(PROTOBUF_VERSION)
 PYTHON_PROTOBUF_SOURCE = $(PROTOBUF_SOURCE)
 PYTHON_PROTOBUF_SITE = $(PROTOBUF_SITE)
 PYTHON_PROTOBUF_LICENSE = BSD-3c
-PYTHON_PROTOBUF_LICENSE_FILES = COPYING.txt
+PYTHON_PROTOBUF_LICENSE_FILES = LICENSE
 PYTHON_PROTOBUF_DEPENDENCIES = host-protobuf
 PYTHON_PROTOBUF_SETUP_TYPE = setuptools
 PYTHON_PROTOBUF_SUBDIR = python