MSVC Projects: Use DOS/Windows Line Feeds for .sln Files

Visual Studio expects .sln files to have DOS/Windows line feeds so that
the their versions and meta data can be properly recognized.

	* MSVC_Net2005/gtkmm.sln:
	* MSVC_Net2008/gtkmm.sln:
	* MSVC_Net2010/gtkmm.sln: Use DOS/Windows line feeds as this is
	  what the Visual Studio IDE expects of those files.