merge_libs.py: fixes Windows breakage: there should be no space after "lib /OUT:".

TBR=andrew@webrtc.org
BUG=b/15773179

Review URL: https://webrtc-codereview.appspot.com/16999004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6793 4adac7df-926f-26a2-2b94-8c16560cd09d