pushover.pl: fix perl warning

The warnings were:

warning: CGI will be removed from the Perl core distribution in the
next major release. Please install it from CPAN. It is being used
at /tmp/weechat/perl/pushover.pl, line 21.

warning: CGI::Util will be removed from the Perl core distribution
in the next major release. Please install it from CPAN. It is being
used at /usr/share/perl5/core_perl/CGI.pm, line 29.