package/mono-gtksharp3: fix build with latest mono

Fix build failure with latest mono which is raised since commit
4c8a285b

Fixes:
 - http://autobuild.buildroot.org/results/ff3edfe406f00a29429d2fa4258af12c87e5fb96



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>