QmlJS: Fix inconsistent copy ctor/operator=

Detected by GCC9.

Change-Id: Ieab7c13c6d66b99cc679c3ac5d4a3da67bcd7767
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/libs/qmljs/qmljsinterpreter.h

@@ -1038,6 +1038,7 @@ class QMLJS_EXPORT Import {
 public:
     Import();
     Import(const Import &other);
+    Import &operator=(const Import &other);
 
     // const!
     ObjectValue *object;