Fix build with gcc-16 (deadbeef DDB_API_LEVEL >= 11 / >= 17).

Two forward-compat diagnostics gcc-16 now treats as hard errors:
  * ddb_waveformat_t.is_bigendian was replaced by `flags` at
    DDB_API_LEVEL 17 (deadbeef.h); drop the initializer -- it set the
    default (0 / little-endian) anyway.
  * the "Remove Waveform From Cache" action callback used the pre-API-11
    `int ctx` signature; DB_plugin_action_callback2_t now takes
    ddb_action_context_t.

Upstream last release 0.5; carried locally.
--- a/waveform.c
+++ b/waveform.c
@@ -1314,7 +1314,6 @@
                 .samplerate = fileinfo->fmt.samplerate,
                 .channelmask = fileinfo->fmt.channelmask,
                 .is_float = 1,
-                .is_bigendian = 0
             };
 
             int eof = 0;
@@ -1723,7 +1722,7 @@
 }
 
 static int
-waveform_action_lookup (DB_plugin_action_t *action, int ctx)
+waveform_action_lookup (DB_plugin_action_t *action, ddb_action_context_t ctx)
 {
     DB_playItem_t *it = NULL;
     deadbeef->pl_lock ();