Category required in bug_actiongroup_page.php

Following-up on making the field required when reporting a new issue
(see issue #26686), we can leverage the new behavior of the "(select)"
OPTION by making the SELECT required to force user to select a value
when performing a batch update of the category.

bug_actiongroup_page.php

@@ -303,7 +303,7 @@
 				'" value="' . $t_date_to_display . '" />';
 			echo '<i class="fa fa-calendar fa-xlg datetimepicker"></i>';
 		} else {
-			echo '<select name="' . $t_form . '" class="input-sm">';
+			echo '<select name="' . $t_form . '" class="input-sm" required>';
 
 			switch( $f_action ) {
 				case 'COPY':