Fix build with gcc-16.

deadbeef's action callback2 takes ddb_action_context_t (not const int)
as of DDB_API_LEVEL 11; gcc-16 hard-errors the incompatible-pointer-types
assignment. Correct the three action callback signatures. Upstream
inactive since 2017.
--- a/opus.c
+++ b/opus.c
@@ -1229,13 +1229,13 @@
         error("save_file: failed to write file %s: code %d\n", outname, res);
 }
 
-static int opus_embed_pics(DB_plugin_action_t *action, const int ctx)
+static int opus_embed_pics(DB_plugin_action_t *action, ddb_action_context_t ctx)
 {
     for_all_selected(embed_album_art);
     return 0;
 }
 
-static int opus_save_streams(DB_plugin_action_t *action, const int ctx)
+static int opus_save_streams(DB_plugin_action_t *action, ddb_action_context_t ctx)
 {
     char pattern[PATH_MAX];
     deadbeef->conf_get_str("opus.opusname", "out.opus", pattern, PATH_MAX-1);
@@ -1243,7 +1243,7 @@
     return 0;
 }
 
-static int opus_save_links(DB_plugin_action_t *action, const int ctx)
+static int opus_save_links(DB_plugin_action_t *action, ddb_action_context_t ctx)
 {
     char pattern[PATH_MAX];
     deadbeef->conf_get_str("opus.oggname", "out.ogg", pattern, PATH_MAX-1);