Bug 19069: (follow-up) Remove perlcritic error

Subroutine prototypes used at line 6, column 1.  See page 194 of PBP.
(Severity: 5)

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>

Koha/MoreUtils.pm

@@ -3,8 +3,7 @@ package Koha::MoreUtils;
 use Modern::Perl;
 
 # From  List::MoreUtils v4.0
-sub singleton (@)
-{
+sub singleton {
     my %seen = ();
     my $k;
     my $seen_undef;