Removes mismatching signs in signal_processing_unittests

Negative inputs was used in WebRtcSpl_NormU32() causing warnings.

BUG=3674
TESTED=locally and trybots
R=tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/17089004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6870 4adac7df-926f-26a2-2b94-8c16560cd09d