[dotnet-linker] Fix whitespace.

tools/dotnet-linker/Steps/AttributeIteratorBaseStep.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections;
 
 using Mono.Linker;

tools/dotnet-linker/Steps/StoreAttributesStep.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 
 using Mono.Cecil;