fix the following 'make includecheck' warning:

  drivers/video/console/vgacon.c: linux/slab.h is included more than once.
Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Cc: Martin Mares <mj@ucw.cz>
Cc: mchehab@infradead.org
Cc: Sam Ravnborg <sam@ravnborg.org>
LKML-Reference: <1247067624.4382.88.camel@ht.satnam>