sql: increment rowcount of FK alteration

Before this patch SQL statement which involves FK constraints creation
or drop didn't increment rowcount:

box.execute("ALTER TABLE t ADD CONSTRAINT fk1 FOREIGN KEY (b) REFERENCES parent (a);")
---
- rowcount: 0
...

This patch fixes this misbehaviour: accidentally VDBE was forgotten to
enable counting changes during ALTER TABLE ADD/DROP constraint.

Closes #4130