fix untracked content in SDL2/external/SDL2

[michaelknoch]

Its super annoying to have things dirty in version control when opening UIKit in Xcode.

HEAD detached at 5d7cfcca3
Untracked files:
  (use "git add <file>..." to include in what will be committed)

	Xcode/SDL/SDL.xcodeproj/xcuserdata/

nothing added to commit but untracked files present (use "git add" to track)

[cshg]

fixed with #240