Add [JniTypeSignature] to GenericHolder<T>.

This type mapping isn't *actually* required, but it *is* used in
`JavaVMFixture`, and it confuses people (me!) if things are
inconsistent.

Author: jonpryor

tests/Java.Interop-Tests/Java.Interop/JniTypeManagerTests.cs

@@ -197,6 +197,7 @@ public void GetTypeSignature ()
 		}
 	}
 
+	[JniTypeSignature (JniTypeName)]
 	class GenericHolder<T> : JavaObject {
 		public  const   string  JniTypeName = "net/dot/jni/test/tests/GenericHolder";