Fix build failure.

src/Features/Core/Portable/Diagnostics/Analyzers/NamingStyleDiagnosticAnalyzerBase.cs

@@ -6,6 +6,7 @@
 using System.Collections.Immutable;
 using System.Linq;
 using Microsoft.CodeAnalysis.CodeStyle;
+using Microsoft.CodeAnalysis.NamingStyles;
 using Microsoft.CodeAnalysis.Simplification;
 using Roslyn.Utilities;