C++: Initialize BaseClass::_isVariadic

Change-Id: I35fa5c9cbc1aa2b5ed46c6e1c0c003fd81bcc1dd
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>
Reviewed-by: André Hartmann <aha_1980@gmx.de>

src/libs/3rdparty/cplusplus/Symbols.h

@@ -503,7 +503,7 @@ protected:
     virtual void visitSymbol0(SymbolVisitor *visitor);
 
 private:
-    bool _isVariadic;
+    bool _isVariadic = false;
     bool _isVirtual;
     FullySpecifiedType _type;
 };