glib: reference .NET 3.5's System.Core in MSBuild project

glib.csproj needs to reference System.Core from .NET 3.5 because
glib-sharp now[1] uses System.Action [2] delegate (not to confuse
with System.Action<T>) which was not available in .NET 2.0's System
reference. This way MonoDevelop stops marking "Action" in red color.

[1] https://github.com/mono/gtk-sharp/commit/8e07e7d2257b185866dedfda9cbf32b2e00cd3ee
[2] http://msdn.microsoft.com/en-us/library/system.action%28v=vs.90%29.aspx