tvheadend: remove -Werror from CFLAGS

Remove -Werror from CFLAGS to avoid build failures caused by warnings.

Fixes the following autobuild failure on MIPS64 n32/64:

   http://autobuild.buildroot.net/results/6be/6be075726ac6db0a48dd6b535841ada932c9ffbb/



Reviewed-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>