Fix mistake in rtp/rtcp/BUILD.gn introduced with r6804.

TEST=buildtools/linux64/gn gen out/Default
TBR=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/21999004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6805 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/webrtc/modules/rtp_rtcp/BUILD.gn b/webrtc/modules/rtp_rtcp/BUILD.gn
index a7499bc..09fa3ac 100644
--- a/webrtc/modules/rtp_rtcp/BUILD.gn
+++ b/webrtc/modules/rtp_rtcp/BUILD.gn
@@ -79,10 +79,10 @@
     "source/rtp_sender_video.cc",
     "source/rtp_sender_video.h",
     "source/video_codec_information.h",
-    'source/rtp_format.cc',
-    'source/rtp_format.h',
-    'source/rtp_format_h264.cc',
-    'source/rtp_format_h264.h',
+    "source/rtp_format.cc",
+    "source/rtp_format.h",
+    "source/rtp_format_h264.cc",
+    "source/rtp_format_h264.h",
     "source/rtp_format_vp8.cc",
     "source/rtp_format_vp8.h",
     "source/rtp_format_video_generic.h",