webkit-efl: add dependency on gstreamer

* fails without it:
  | -- checking for module 'gstreamer-0.10'
  | --   package 'gstreamer-0.10' not found
  | -- checking for module 'gstreamer-base-0.10'
  | --   package 'gstreamer-base-0.10' not found
  | -- checking for module 'gstreamer-app-0.10'
  | --   package 'gstreamer-app-0.10' not found
  | -- checking for module 'gstreamer-audio-0.10'
  | --   package 'gstreamer-audio-0.10' not found
  | -- checking for module 'gstreamer-fft-0.10'
  | --   package 'gstreamer-fft-0.10' not found
  | -- checking for module 'gstreamer-interfaces-0.10'
  | --   package 'gstreamer-interfaces-0.10' not found
  | -- checking for module 'gstreamer-pbutils-0.10'
  | --   package 'gstreamer-pbutils-0.10' not found
  | -- checking for module 'gstreamer-video-0.10'
  | --   package 'gstreamer-video-0.10' not found
  | CMake Error at cmake-2.8/Modules/FindPackageHandleStandardArgs.cmake:108 (message):
  |   Could NOT find GStreamer (missing: GSTREAMER_INCLUDE_DIRS

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>