Prevent this database deadlock:

1. bulk scan begins transaction, locking the db
2. cache regeneration is triggered, but waits for db to unlock
3. bulk scan calls cache_artwork_ping, which can't return because cache thread is waiting
-> scan thread is waiting for cache thread, which is waiting for scan thread

src/cache.h

@@ -30,7 +30,7 @@ cache_daap_threshold(void);
 #define CACHE_ARTWORK_GROUP 0
 #define CACHE_ARTWORK_INDIVIDUAL 1
 
-int
+void
 cache_artwork_ping(char *path, time_t mtime, int del);
 
 int