network-manager: adjust license info

libgsystem is gone, so remove that section. Fixes:
http://autobuild.buildroot.net/results/4b9/4b9826f540b8da43c0434769ad9485529cf762e6/



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