Add include stdlib.h to files using abs.

abs function is declared in stdlib.h

Committing for alextaran@chromium.org.
Reviewed here: http://review.webrtc.org/4239004/

TBR=mflodman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/4349004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@5170 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/rtp_rtcp/source/forward_error_correction.cc

@@ -11,6 +11,7 @@
 #include "webrtc/modules/rtp_rtcp/source/forward_error_correction.h"
 
 #include <assert.h>
+#include <stdlib.h>
 #include <string.h>
 
 #include <algorithm>

webrtc/video_engine/stream_synchronization.cc

@@ -12,6 +12,7 @@
 
 #include <assert.h>
 #include <math.h>
+#include <stdlib.h>
 
 #include <algorithm>