Always install networkmanager.png icons regardless which backend is

going to be compiled (networkmanager-0.7 or networkmanager_fake-0.9).
networkmanager_fake-0.9 does not install any icon, then when using
Plasma NM from branch nm09 there is no icon for it. Plasma NM's master
branch currently installs the icons, I will remove them from master
branch to prevent conflicts with this commit.

CCBUG: 293711
(cherry picked from commit e5a73150)