package/mono: needs C++

mono needs a working C++ compiler to get the exact type of size_t since
version 5.16.0.179 and
https://github.com/mono/mono/commit/d8af77551de1f59803a46946786e0f9a5eb5cb9d

Fixes:
 - http://autobuild.buildroot.org/results/ee6d037154127cc6572f06e9a78bae383ea381bc



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Tested-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>