display oncevar loplugin by default

seems to be annoying some people. I'll run this one myself, and at some
stage create some code for enabling a subset of extra plugins via
configure.ac

Change-Id: Ia95701f63f65751d75b5a3fecffb1fc1a82f38e0

compilerplugins/clang/oncevar.cxx

@@ -404,7 +404,7 @@ bool OnceVar::VisitDeclRefExpr( const DeclRefExpr* declRefExpr )
     return true;
 }
 
-loplugin::Plugin::Registration< OnceVar > X("oncevar", true);
+loplugin::Plugin::Registration< OnceVar > X("oncevar", false);
 
 }