fix crash in refcounting plugin

introduced by commit c15b4cf39a
"Adapt to current Clang trunk towards 3.7"

Change-Id: I00f58d3bc79e641df9bba4e9b1d5c8463b87dc42

compilerplugins/clang/refcounting.cxx

@@ -90,7 +90,7 @@ bool isDerivedFrom(const CXXRecordDecl *decl, const char *pString) {
                 { return BaseCheckNotSubclass(
                         BaseDefinition, const_cast<char *>(pString)); },
 #endif
-            true))
+            static_cast<void*>(const_cast<char*>(pString)), true))
     {
         return true;
     }