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] 8e07e7d225
[2] http://msdn.microsoft.com/en-us/library/system.action%28v=vs.90%29.aspx

glib/glib.csproj

@@ -88,4 +88,7 @@
     <Compile Include="VariantType.cs" />
     <Compile Include="GLibSynchronizationContext.cs" />
   </ItemGroup>
+  <ItemGroup>
+    <Reference Include="System.Core" />
+  </ItemGroup>
 </Project>