ffmpeg: fix screen sharing on GNU/Linux

ffmpeg must be configured with --enable-indev=x11grab_xcb for the
x11grab (for screen sharing) to work

Change-Id: Iec6010dd4854d7b6d043c294be4aaa6215a27851
Tuleap: #1146

contrib/src/ffmpeg/rules.mak

@@ -121,7 +121,7 @@ DEPS_ffmpeg = iconv zlib x264 vpx opus speex $(DEPS_vpx)
 ifdef HAVE_LINUX
 FFMPEGCONF += --target-os=linux --enable-pic
 ifndef HAVE_ANDROID
-FFMPEGCONF += --enable-indev=v4l2 --enable-indev=x11grab --enable-x11grab
+FFMPEGCONF += --enable-indev=v4l2 --enable-indev=x11grab_xcb --enable-indev=x11grab --enable-x11grab
 else
 # used to avoid Text Relocations
 FFMPEGCONF += --extra-cxxflags=-fPIC --extra-cflags=-fPIC