Fix building erofs-utils against musl

Add a missing header include to fix building erofs-utils against
musl libc.

Bug: 190084016
Test: m USE_HOST_MUSL=true host-native
Change-Id: I15d4f4235e83e08d4d974b3fa0e07a2ad8ba609e
diff --git a/lib/dir.c b/lib/dir.c
index e53c897..46805b4 100644
--- a/lib/dir.c
+++ b/lib/dir.c
@@ -2,6 +2,7 @@
 #include "erofs/print.h"
 #include "erofs/dir.h"
 #include <stdlib.h>
+#include <sys/stat.h>
 
 static int traverse_dirents(struct erofs_dir_context *ctx,
 			    void *dentry_blk, unsigned int lblk,