sql: don't build sql as a separate library

SQL heavily depends on box, and box on SQL. So they can't be
separate libraries. The build started failing with undefined box
symbols in SQL, when code of the latter has slightly changed in
one of the recent commits.

The build failed only with UB sanitizer enabled, but
'VERBOSE=1 make' showed that both with UB and without UB the build
command was the same (not counting -fsanitize flags). So the
sanitizer has nothing to do with it.

The patch makes SQL sources being built as a part of box library.

Closes #5067