directory: moved dirvec struct declaration to dirvec.h

No idea why it was created in directory.h, but it should be in
dirvec.h.

src/directory.h

@@ -19,6 +19,7 @@
 #ifndef DIRECTORY_H
 #define DIRECTORY_H
 
+#include "dirvec.h"
 #include "songvec.h"
 
 #include <stdbool.h>
@@ -33,11 +34,6 @@
 #define DIRECTORY_MPD_VERSION	"mpd_version: "
 #define DIRECTORY_FS_CHARSET	"fs_charset: "
 
-struct dirvec {
-	struct directory **base;
-	size_t nr;
-};
-
 struct directory {
 	char *path;
 	struct dirvec children;