Fix build with gcc-16 and deadbeef DDB_API_LEVEL >= 11.

deadbeef's DB_plugin_action_callback2_t changed its second parameter from
`int` to `ddb_action_context_t` at DDB_API_LEVEL 11 (deadbeef.h). gcc-16
promotes the resulting incompatible-pointer-types assignment to a hard
error; match the current callback2 signature.

Upstream is unmaintained (last commit 2014); carried locally.
--- a/rg_control.c
+++ b/rg_control.c
@@ -72,7 +72,7 @@
 
 
 static int
-replaygain_control_action_callback(DB_plugin_action_t *action, int ctx) {
+replaygain_control_action_callback(DB_plugin_action_t *action, ddb_action_context_t ctx) {
     on_button_config (NULL);
     return 0;
 }