[system.data] updated the makefile to reference system.core
 - the type System.Runtime.CompilerServices.ExtensionAttribute is needed for extension methods
 - it exists in System.Core for 3.5 and 4.0
 - it has been moved to corlib for 4.5
 - NOTE: it actually exists in 2.0 and 3.0 as well in System.Core in MONO due to compiler directives, but not in MS.NET.