Fix unescaped left brace warning in monster regex.

Fix unescaped left brace warning in monster regex.

New versions of Perl are starting to complain about unescaped braces
in regular expressions, and supposedly the warning will become a fatal
error in Perl 5.30. This particular warning is,

  Unescaped left brace in regex is deprecated, passed through in regex;
  marked by <-- HERE in m/^(CLEAN|SPAM(?:MY)?|INFECTED \(.*?\)|BANNED \(.*?\)
  |BAD-HEADER(?:-\d)?|UNCHECKED|MTA-BLOCKED|OVERSIZED|OTHER|TEMPFAIL)(?:
  { <-- HERE [^}]+})?, ([^[]+ )?(?:([^<]+) )?[<(](.*?)[>)] -> ([(<].*?[)>]),
  (?:.*Hits: ([-+.\d]+))(?:.* size: (\d+))?(?:.* autolearn=(\w+))?/ at
  /usr/bin/amavis-logwatch line 2286.

and it was fixed by going to line 2286 and putting a backslash before
the left brace.