sdl2-compat.pc.in needing some dependencies

Because sdl2-compat's .h files use X11:

```
include/SDL2/SDL_syswm.h:#include <X11/Xlib.h>
include/SDL2/SDL_syswm.h:#include <X11/Xatom.h>
include/SDL2/SDL_egl.h:#include <X11/Xlib.h>
include/SDL2/SDL_egl.h:#include <X11/Xutil.h>
```

`sdl2-compat.pc.in` should be augmented by a line:

```
Requires: xproto x11
```
