C#: Update the DB scheme, such that compound assignment operator calls can be considered qualifiable expressions.

csharp/ql/lib/semmlecode.csharp.dbscheme

@@ -1338,7 +1338,8 @@ dynamic_member_name(
 
 @qualifiable_expr = @member_access_expr
                   | @method_invocation_expr
-                  | @element_access_expr;
+                  | @element_access_expr
+                  | @assign_op_call_expr;
 
 conditional_access(
   unique int id: @qualifiable_expr ref);