ubifs: fix checkpatch warning

Fix the following checkpatch warning:-

WARNING: externs should be avoided in .c files

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
diff --git a/fs/ubifs/ubifs.h b/fs/ubifs/ubifs.h
index 633631e..2213201 100644
--- a/fs/ubifs/ubifs.h
+++ b/fs/ubifs/ubifs.h
@@ -2137,6 +2137,13 @@
 int ubifs_decompress(const void *buf, int len, void *out, int *out_len,
 		     int compr_type);
 
+/* these are used in cmd_ubifs.c */
+int ubifs_init(void);
+int ubifs_mount(char *vol_name);
+void ubifs_umount(struct ubifs_info *c);
+int ubifs_ls(char *dir_name);
+int ubifs_load(char *filename, u32 addr, u32 size);
+
 #include "debug.h"
 #include "misc.h"
 #include "key.h"