fs: d_path: include internal.h

make W=1 warns about a missing prototype that is defined but
not visible at point where simple_dname() is defined:

fs/d_path.c:317:7: error: no previous prototype for 'simple_dname' [-Werror=missing-prototypes]

Signed-off-by: Arnd Bergmann <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

fs/d_path.c

···
 #include <linux/slab.h>
 #include <linux/prefetch.h>
 #include "mount.h"
+#include "internal.h"
 
 struct prepend_buffer {
 	char *buf;