disable flatten loplugin

<noelgrandin> sberg, new plugin flatten just went active
<sberg> noelgrandin, with a measure to avoid extending lifetime of (problematic) local vars?
<noelgrandin> sberg, no
<sberg> noelgrandin, how can you make it active then?
<noelgrandin> sberg, ok, will disable

Change-Id: I595d1a50ff34417faf73b777714f9dc92e2a43d2

compilerplugins/clang/flatten.cxx

@@ -344,7 +344,7 @@ std::string Flatten::getSourceAsString(SourceRange range)
     return std::string( p1, p2 - p1);
 }
 
-loplugin::Plugin::Registration< Flatten > X("flatten", true);
+loplugin::Plugin::Registration< Flatten > X("flatten", false);
 
 }