python-pyasn: fix license file

It's not LICENSE.txt and not LICENSE, fixes:
http://autobuild.buildroot.net/results/d29/d293f35d8034eaaa519565b14641d1b56206cbc8/

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

package/python-pyasn/python-pyasn.mk

@@ -8,7 +8,7 @@ PYTHON_PYASN_VERSION = 0.1.9
 PYTHON_PYASN_SOURCE = pyasn1-$(PYTHON_PYASN_VERSION).tar.gz
 PYTHON_PYASN_SITE = https://pypi.python.org/packages/source/p/pyasn1
 PYTHON_PYASN_LICENSE = BSD-2c
-PYTHON_PYASN_LICENSE_FILES = LICENSE
+PYTHON_PYASN_LICENSE_FILES = LICENSE.txt
 PYTHON_PYASN_SETUP_TYPE = distutils
 
 $(eval $(python-package))