t/run/exit.t: Fix syntax of 'my'

I found this by code reading

Author: khwilliamson

t/run/exit.t

@@ -45,7 +45,7 @@ plan(tests => $numtests);
 my $native_success = 0;
    $native_success = 1 if $^O eq 'VMS';
 
-my $exit, $exit_arg;
+my ($exit, $exit_arg);
 
 $exit = run('exit');
 is( $exit >> 8, 0,              'Normal exit' );