n_tty: Fix build breakage on ppc64

Commit 20bafb3d


  '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>