Add light gun option for Genesis gamepads

seems to work; why do we have our own enum instead of using one of GPGX'?

src/BizHawk.Emulation.Cores/Consoles/Sega/gpgx64/GPGX.cs

@@ -145,6 +145,8 @@ namespace BizHawk.Emulation.Cores.Consoles.Sega.gpgx
 					return LibGPGX.INPUT_SYSTEM.SYSTEM_WAYPLAY;
 				case ControlType.Mouse:
 					return LibGPGX.INPUT_SYSTEM.SYSTEM_MOUSE;
+				case ControlType.LightPhaser:
+					return LibGPGX.INPUT_SYSTEM.SYSTEM_LIGHTPHASER;
 			}
 		}
 
@@ -173,7 +175,8 @@ namespace BizHawk.Emulation.Cores.Consoles.Sega.gpgx
 			Activator,
 			Teamplayer,
 			Wayplay,
-			Mouse
+			Mouse,
+			LightPhaser,
 		}