Fix build with gcc-16.

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

Upstream last commit 2022; carried locally.
--- a/spectrum.c
+++ b/spectrum.c
@@ -120,7 +120,7 @@
 }
 
 static void
-spectrum_wavedata_listener (void *ctx, ddb_audio_data_t *data) {
+spectrum_wavedata_listener (void *ctx, const ddb_audio_data_t *data) {
     w_spectrum_t *w = ctx;
     g_assert (w->data->samples != NULL);