package/mono: fixing missing locale

This patch fixes compilation error in case of a missing locale in toolchain.

Fixes: http://autobuild.buildroot.net/results/a0fc33a4ed90844f9529acc76a6ba183fe253771/
       http://autobuild.buildroot.net/results/3ff839e894f0b8f9c2e3af9629d45e8efc09aecd/
       http://autobuild.buildroot.net/results/5e11117048d965bc1fc44c738bb51f11164304af/



Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>