Fix Check with StyleCop menu not returning any StyleCop warnings.

src/Main/SharpDevelop/Project/Build/MSBuildEngine/MSBuildEngine.cs

@@ -94,6 +94,7 @@ namespace ICSharpCode.SharpDevelop.Project
 				throw new ArgumentNullException("feedbackSink");
 			
 			var additionalTargetFileList = additionalTargetFiles != null ? additionalTargetFiles.ToList() : new List<string>();
+			additionalTargetFileList.AddRange(AdditionalTargetFiles.Select(fileName => fileName.ToString()));
 			if (project.MinimumSolutionVersion >= SolutionFormatVersion.VS2010) {
 				additionalTargetFileList.Add(Path.Combine(Path.GetDirectoryName(typeof(MSBuildEngine).Assembly.Location), "SharpDevelop.TargetingPack.targets"));
 			}