Emit dummy statements for probes.d probes when disabled

Emit dummy statements for probes.d probes when disabled

When building without --enable-dtrace, emit dummy

    do {} while (0)

statements for the stubbed-out TRACE_POSTGRESQL_foo() macros
instead of empty macros that totally elide the original probe
statement.

This fixes the

    warning: suggest braces around empty body in an ‘if’ statement [-Wempty-body]

introduced by b94409a02f.

Author: Craig Ringer <craig.ringer@2ndquadrant.com>
Discussion: https://www.postgresql.org/message-id/flat/20210504221531.cfvpmmdfsou6eitb%40alap3.anarazel.de