When using v2.0.2 of the subproject, it triggers werror for
unused functions that come from the fdkaac headers.
This avoids errors like the following when werror is set.
```
subprojects/fdk-aac-2.0.2/fdk-aac/FDK_audio.h:757:29: error: ‘FDKlibInfo_lookup’
defined but not used [-Werror=unused-function]
757 | static FDK_AUDIO_INLINE INT FDKlibInfo_lookup(const LIB_INFO* info,
```
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6611>