package/freerdp: can not build on uClibc

FreeRDP uses eventfd_{read,write}, which is not supported in uClibc.

Fixes:
    http://autobuild.buildroot.net/results/ad7/ad7ade4c436d21cba930f40c613870fbadec840e/
    http://autobuild.buildroot.net/results/5a3/5a30aecbb426f76a22e8285931a0a8cd59b5e70b/
    http://autobuild.buildroot.net/results/6e0/6e0de0538e95ae156f7d2ad6eede46e44be81c8e/


    [...]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>