package/weston: mark the RDP backend as broken

weston needs FreeRDP from master, as it uses new settings only
introduced in there.

Since it is too late in the release cycle to bump FreeRDP, just mark the
weston RDP backend as broken.

Fixes:
    http://autobuild.buildroot.org/results/ef6/ef685a41a56e81dc93a40f23f8cd55860f44e055/
    http://autobuild.buildroot.org/results/c42/c42d72498e05364a718748caef74bba6c423d7d1/
    http://autobuild.buildroot.org/results/c78/c783695a5b1a18976361ea36627505b281b11781/


    ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>