Add `sdl2.pc` pkg-config file for maximum backwards compatibility

The existing file sdl2_compat.pc contains Provides: sdl2 = 2.32.50, but this isn't recognised by all tooling. For example, to build the Haskell sdl2 and sdl2-ttf packages I've had to do sudo ln -s /usr/lib/pkgconfig/sdl2_compat.pc /usr/lib/pkgconfig/sdl2.pc.

I'll report this to Cabal, the Haskell build tool (EDIT: done), but in the meantime perhaps we should just provide a backwards-compatible pkgconfig file? I see no real downside, and Debian already does this.