package/rpi-userland: drop -Werror

Fixes:
    http://autobuild.buildroot.org/results/501/5012affec6c85fe7749ac86d18d3c191f4e74bdf/



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