mosquitto: disable with broken NIOS 2 toolchains

mosquitto triggers the infamous _gp issue with the NIOS 2 toolchains,
so let's not allow mosquitto in such situations.

Fixes:

  http://autobuild.buildroot.net/results/b853369452115b0c6f32c6c960af2dbdf71a74af/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>