FS-7513: fix compile w/o openal

src/mod/applications/mod_conference/mod_conference.c

@@ -301,6 +301,7 @@ typedef struct al_handle_s {
 #else 
 typedef struct al_handle_s {
 	int unsupported;
+	switch_mutex_t *mutex;
 } al_handle_t;
 #endif