Add an opaque pointer to watch_info

Used on FreeBSD to hold a pointer to the kevent struct.

src/db.h

@@ -200,6 +200,10 @@ struct watch_info {
   char *path;
   uint32_t cookie;
   int libidx;
+
+#if defined(__FreeBSD__)
+  void *priv;
+#endif
 };
 
 #define wi_offsetof(field) offsetof(struct watch_info, field)