Disable loplugin:casttovoid for now

I'd accidentally developed it against a Clang built without
LLVM_ENABLE_ASSERTIONS=ON, so only saw issues with
containsPreprocessingConditionalInclusion() now.

Change-Id: I5da6775a06de748e78b34a42f63b0543c3893124

compilerplugins/clang/casttovoid.cxx

@@ -487,7 +487,7 @@ private:
     }
 };
 
-static loplugin::Plugin::Registration<Visitor> reg("casttovoid");
+static loplugin::Plugin::Registration<Visitor> reg("casttovoid", false);
 
 }