VS: Convert include path to backslashes for VS >= 10

In VS <= 9 we already convert include paths to use backslashes
but the conversion was left out of the VS >= 10 generator.
Add the conversion now for consistency.  VS 11 and 12 now require
it because they mistakenly interpret forward-slash options that
happen to occur in paths:

 http://connect.microsoft.com/VisualStudio/feedback/details/778367

Source/cmVisualStudio10TargetGenerator.cxx

@@ -1420,7 +1420,9 @@ OutputIncludes(std::vector<std::string> const & includes)
   for(std::vector<std::string>::const_iterator i =  includes.begin();
       i != includes.end(); ++i)
     {
-    *this->BuildFileStream << cmVS10EscapeXML(*i) << ";";
+    std::string incDir = *i;
+    this->ConvertToWindowsSlash(incDir);
+    *this->BuildFileStream << cmVS10EscapeXML(incDir) << ";";
     }
   this->WriteString("%(AdditionalIncludeDirectories)"
                     "</AdditionalIncludeDirectories>\n", 0);