kms_flip: Add -lrt back

kms_flip: Add -lrt back

kms_flip doesn't build due to missing -lrt.

The build was broken due to a typo in this commit:

commit acdf7171264a5139caabb78127c9eef5080c0682
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sat Dec 15 00:48:47 2012 +0100

    tests: s/flip_test/kms_flip

    OCD wins!

    Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
For some reason libudev.pc brings in -lrt as well, which probably
caused the typo to go unnoticed.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>