drivers/tty/serial/pch_uart.c needs slab.h

drivers/tty/serial/pch_uart.c needs slab.h

alpha allmodconfig:

  drivers/tty/serial/pch_uart.c: In function 'dma_handle_tx':
  drivers/tty/serial/pch_uart.c:873: error: implicit declaration of function 'kzalloc'
  drivers/tty/serial/pch_uart.c:873: warning: assignment makes pointer from integer without a cast
  drivers/tty/serial/pch_uart.c: In function 'pch_uart_init_port':
  drivers/tty/serial/pch_uart.c:1403: warning: assignment makes pointer from integer without a cast

Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>