Fix leak caused by missing virtual destructor

Author: nyalldawson

python/core/qgsexpressioncontextgenerator.sip

@@ -38,4 +38,6 @@ class QgsExpressionContextGenerator
      * and return an expression context.
      */
     virtual QgsExpressionContext createExpressionContext() const = 0;
+
+    virtual ~QgsExpressionContextGenerator();
 };

src/core/qgsexpressioncontextgenerator.h

@@ -43,6 +43,8 @@ class QgsExpressionContextGenerator
      * @note Added in QGIS 3.0
      */
     virtual QgsExpressionContext createExpressionContext() const = 0;
+
+    virtual ~QgsExpressionContextGenerator() {}
 };
 
 #endif // QGSEXPRESSIONCONTEXTGENERATOR_H