Add missing include.

...and remove an unused one.

Change-Id: I7949964b6d74f861ad07ed56f5092014fdfc7c8a
diff --git a/f2fs_utils/f2fs_utils.c b/f2fs_utils/f2fs_utils.c
index d42ccc1..4c92622 100644
--- a/f2fs_utils/f2fs_utils.c
+++ b/f2fs_utils/f2fs_utils.c
@@ -29,7 +29,7 @@
 #define _LARGEFILE64_SOURCE
 
 #include <fcntl.h>
-#include <dlfcn.h>
+#include <string.h>
 
 #include <f2fs_fs.h>
 #include <f2fs_format_utils.h>