Fix unescaped left brace warning for "Config: {".

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/^Config: { <-- HERE / at ./postfix-logwatch
  line 1850.

and it was fixed by going to line 1850 and putting a backslash before
the left brace.

Bug: https://sourceforge.net/p/logreporters/bugs/4/