n_tty: Fix build breakage on ppc64

Commit 20bafb3d23d108bc0a896eb8b7c1501f4f649b77
  'n_tty: Move buffers into n_tty_data'
broke the ppc64 build.

Include vmalloc.h for the required function declarations.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index ed6c4e4..2c120b6 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -50,6 +50,7 @@
 #include <linux/uaccess.h>
 #include <linux/module.h>
 #include <linux/ratelimit.h>
+#include <linux/vmalloc.h>
 
 
 /* number of characters left in xmit buffer before select has we have room */