Fix build with gcc-16.

Two forward-compat diagnostics gcc-16 now treats as hard errors:
  * vumeter_draw_bars() returns void but bailed out with `return FALSE;`
    on a NULL surface (-Wreturn-mismatch).
  * vis_waveform_listen()'s callback parameter became
    const ddb_audio_data_t * (deadbeef.h); match it in the listener.

Upstream unmaintained since 2017; carried locally.
--- a/vumeter.c
+++ b/vumeter.c
@@ -808,7 +808,7 @@
 }
 
 static void
-vumeter_wavedata_listener (void *ctx, ddb_audio_data_t *data) {
+vumeter_wavedata_listener (void *ctx, const ddb_audio_data_t *data) {
     w_vumeter_t *w = ctx;
     deadbeef->mutex_lock (w->mutex);
     w->channels = MIN (MAX_CHANNELS, data->fmt->channels);
@@ -879,7 +879,7 @@
 
     unsigned char *data = cairo_image_surface_get_data (w->surf);
     if (!data) {
-        return FALSE;
+        return;
     }
     int stride = cairo_image_surface_get_stride (w->surf);
     memset (data, 0, height * stride);