Core/Formations: update codestyle and fix crash

Port From (https://github.com/TrinityCore/TrinityCore/commit/861cf261c5dfc29d6980908ec36ebe7aad68a462)

Source/Framework/Constants/CreatureConst.cs

@@ -13,7 +13,8 @@
  *
  * You should have received a copy of the GNU General Public License
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
- */
+ */
+using System;
 
 namespace Framework.Constants
 {
@@ -365,6 +366,7 @@ namespace Framework.Constants
         Other
     }
 
+    [Flags]
     public enum GroupAIFlags
     {
         None = 0,          // No creature group behavior