package/mono-gtksharp3: fix build with latest mono

Fixes:
 - http://autobuild.buildroot.org/results/7cae5b32285fe0edf69eab3297be9c6a4e76b7b8



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>