contrib/rpm: fix libnm License tag

https://bugzilla.redhat.com/show_bug.cgi?id=1723395
(cherry picked from commit 2752e3a7)
15 jobs for nm-1-20 in 38 minutes and 11 seconds (queued for 1 minute and 16 seconds)
Status Job ID Name Coverage
  Test
passed #461004
checkpatch

00:01:07

manual #461009
allowed to fail manual
t_centos:7.5.1804
passed #461010
t_centos:7.6.1810

00:24:38

passed #461016
t_debian:10

00:19:01

passed #461015
t_debian:9

00:19:05

passed #461018
t_debian:sid

00:18:58

manual #461017
allowed to fail manual
t_debian:testing
passed #461005
t_fedora:28

00:26:11

passed #461006
t_fedora:29

00:36:16

passed #461007
t_fedora:30

00:23:35

manual #461008
allowed to fail manual
t_fedora:rawhide
passed #461011
t_ubuntu:16.04

00:19:43

passed #461012
t_ubuntu:18.04

00:16:35

manual #461014
allowed to fail manual
t_ubuntu:devel
manual #461013
allowed to fail manual
t_ubuntu:rolling