Commit 20bafb3d
  'n_tty: Move buffers into n_tty_data'
broke the ppc64 build.

Include vmalloc.h for the required function declarations.
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>