SDL_gfx: propagate SDL to -dev outputs

`SDL_gfx.pc` refers to `sdl.pc`. In addition to that some `SDL_gfx`
headers import `SDL.h` directly. I noticed it when attempted to build a
small SDL demo project which imports only `SDL_gfx`. It out to have
worked. Instead build fails as:

    $ SDL1_CFLAGS=$(shell pkg-config SDL_gfx --cflags)
    ...
    Package sdl was not found in the pkg-config search path.
    Perhaps you should add the directory containing `sdl.pc'
    to the PKG_CONFIG_PATH environment variable

This change is enough to make it work.

pkgs/development/libraries/SDL_gfx/default.nix

@@ -9,6 +9,7 @@ stdenv.mkDerivation rec {
     sha256 = "0ijljhs0v99dj6y27hc10z6qchyp8gdp4199y6jzngy6dzxlzsvw";
   };
 
+  propagatedBuildInputs = [ SDL ];
   buildInputs = [ SDL ] ;
 
   configureFlags = [ "--disable-mmx" ]