[dotnet-linker] Add a using so that the static registrar sees LinkContext extension methods in the Mono.Linker namespace

tools/common/StaticRegistrar.cs

@@ -20,6 +20,7 @@ using Registrar;
 using Foundation;
 using ObjCRuntime;
 using Mono.Cecil;
+using Mono.Linker;
 using Mono.Tuner;
 
 namespace Registrar {