Revert "Remove unneeded != operator"

MSVC errors when we don't keep this operator.

This reverts commit c6499cf5db.

pcbnew/pad.h

@@ -821,6 +821,7 @@ public:
     double Similarity( const BOARD_ITEM& aOther ) const override;
 
     bool operator==( const BOARD_ITEM& aOther ) const override;
+    bool operator!=( const BOARD_ITEM& aOther ) const { return !operator==( aOther ); }
 
 #if defined(DEBUG)
     virtual void Show( int nestLevel, std::ostream& os ) const override { ShowDummy( os ); }