Ports: Enable MP3 support for SDL2_mixer

`SDL2_mixer` needs `libmpg123` to support MP3 playback. By adding
it as a dependency, its compilation now outputs:

```
checking mpg123.h usability... yes
checking mpg123.h presence... yes
checking for mpg123.h... yes
checking for mpg123_replace_reader_handle in -lmpg123... yes
-- dynamic libmpg123 -> libmpg123.so.0
```

Sound output was tested with
[an implementation](https://gist.github.com/cdave1/10563386) found
online.

Ports/SDL2_mixer/package.sh

@@ -6,7 +6,7 @@ use_fresh_config_sub=true
 config_sub_path=build-scripts/config.sub
 files="https://www.libsdl.org/projects/SDL_mixer/release/SDL2_mixer-${version}.tar.gz SDL2_mixer-${version}.tar.gz b4cf5a382c061cd75081cf246c2aa2f9df8db04bdda8dcdc6b6cca55bede2419"
 auth_type=sha256
-depends=("libmodplug" "libvorbis" "SDL2")
+depends=("libmodplug" "libmpg123" "libvorbis" "SDL2")
 
 configure() {
     export LIBS="-L${SERENITY_INSTALL_ROOT}/usr/local/lib"