Enable some SDL libraries on Darwin

pkgs/development/libraries/SDL2_gfx/default.nix

···
     license = stdenv.lib.licenses.lgpl2Plus;
 
     maintainers = [ stdenv.lib.maintainers.bjg ];
-    platforms = stdenv.lib.platforms.linux;
+    platforms = stdenv.lib.platforms.unix;
   };
 }

pkgs/development/libraries/SDL2_image/default.nix

···
   meta = {
     description = "SDL image library";
     homepage = "http://www.libsdl.org/projects/SDL_image/";
-    platforms = stdenv.lib.platforms.linux;
+    platforms = stdenv.lib.platforms.unix;
   };
 }

pkgs/development/libraries/SDL_gfx/default.nix

···
     license = stdenv.lib.licenses.lgpl2Plus;
 
     maintainers = [ stdenv.lib.maintainers.bjg ];
-    platforms = stdenv.lib.platforms.linux;
+    platforms = stdenv.lib.platforms.unix;
   };
 }