private string IncludePathSDL2
{
get { return Path.GetFullPath(Path.Combine(ThirdPartyPath, "SDL2/SDL2", "include")); }
}
And no other *.Build.cs files under <ProjectDir>\Plugins\JoystickPlugin ?
Also, do you have a file <ProjectDir>\Plugins\JoystickPlugin\ThirdParty\SDL2\SDL2\include\SDL2\SDL.h ?