Add a missing backslash-escape to left brace in a regexp

Perl 5.28 introduced a warning about unescaped left brace in a regexp,
resulting in a warning issued during amavisd startup:

Unescaped left brace in regex is deprecated here (and will be fatal in Perl
5.32), passed through in regex; marked by <-- HERE in m/ ( { <-- HERE }
(?: / \* )? | \* ) / at (eval 112) line 830.

Currently the warning is harmless although annoying, but will become a fatal error
with the next version of perl.

PR: 234173
Submitted by: Mark.Martinec@ijs.si