package/freerdp: fix build with gstreamer support

FreeRDP with gstreamer support enabled needs gstreamer and gst-plugins-base
with gstapp.

Fixes:
  http://autobuild.buildroot.net/results/937/9373d4238bc3f027e571581bd3531018c4faed91/
  http://autobuild.buildroot.org/results/f77/f77a2c9eea7973b54a22e12ab33ee8964adaddfd/



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Tested-by: Gergely Imreh <imrehg@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>