Add missing import

fixes f7b1482

Author: YoshiRulz

src/BizHawk.Client.EmuHawk/RetroAchievements/RCheevos.GameInfo.cs

@@ -4,6 +4,7 @@
 using System.Linq;
 using System.Text;
 
+using BizHawk.Common;
 using BizHawk.Common.CollectionExtensions;
 
 namespace BizHawk.Client.EmuHawk