Fix build with gcc-16.

deadbeef's vis_waveform_listen() callback parameter became
`const ddb_audio_data_t *` (deadbeef.h); match it or gcc-16 errors on
the incompatible pointer type. Add the const.

Upstream unmaintained since 2015; carried locally.
--- a/spectrogram.c
+++ b/spectrogram.c
@@ -603,7 +603,7 @@
 }
 
 static void
-spectrogram_wavedata_listener (void *ctx, ddb_audio_data_t *data) {
+spectrogram_wavedata_listener (void *ctx, const ddb_audio_data_t *data) {
     w_spectrogram_t *w = ctx;
     if (!w->samples) {
         return;