add pc2 extension for wonderswan

these are pocket challenge v2 roms, which is some handheld system which is actually just a wonderswan inside so these roms work anyways with cygne

src/BizHawk.Client.Common/RomLoader.cs

@@ -854,7 +854,7 @@ namespace BizHawk.Client.Common
 
 			public static readonly IReadOnlyCollection<string> VEC = new[] { "vec" };
 
-			public static readonly IReadOnlyCollection<string> WSWAN = new[] { "ws", "wsc" };
+			public static readonly IReadOnlyCollection<string> WSWAN = new[] { "ws", "wsc", "pc2" };
 
 			public static readonly IReadOnlyCollection<string> ZXSpectrum = new[] { "tzx", "tap", "dsk", "pzx" };
 

src/BizHawk.Emulation.Common/Database/Database.cs

@@ -346,6 +346,7 @@ namespace BizHawk.Emulation.Common
 
 				case ".WS":
 				case ".WSC":
+				case ".PC2"
 					game.System = VSystemID.Raw.WSWAN;
 					break;