Fix warning in BMW_E65::GetGear

Teh parameter outGear is currently unused so mark it
as such to stop the compiler moaning.

src/BMW_E65.cpp

@@ -371,7 +371,7 @@ void BMW_E65::Engine_Data()
 
 }
 
-bool BMW_E65::GetGear(Vehicle::gear& outGear)
+bool BMW_E65::GetGear(Vehicle::gear& outGear __attribute__((unused)))
 {
     return false;
 }