Fix webkit on Ubuntu

WARNING("Plugin"): LLPluginInstance::load: apr_dso_load of
*/viewer-linux-i686-releasesse2/newview/packaged/bin/llplugin/libmedia_plugin_webkit.so
failed with error 20019 , additional info string:
*/viewer-linux-i686-releasesse2/newview/packaged/bin/llplugin/libmedia_plugin_webkit.so:
undefined symbol: XSyncIntsToValue

Apparently our webkit package is underlinked, it doesn't
include libXext which is needed for symbol XSyncIntsToValue.
Both, Fritigern and Inusaito had this problem on Ubuntu.
This patch adds the library to the webkit plugin as a work-
around.

indra/cmake/WebKitLibPlugin.cmake

@@ -76,6 +76,7 @@ elseif (LINUX)
         fontconfig
         X11
         Xrender
+        Xext
         GL
         )
      endif (STANDALONE)