package/freerdp: fix gstreamer-1.x detection

Backport a patch from upstream.

Partially fixes:
    http://autobuild.buildroot.org/results/81aa66ddd88919295ccb5f34b527b737627263a7/



Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>