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>