staging: comedi: comedi_buf: needs <linux/vmalloc.h>

As reported by the kbuild test robot:

Commit: ce157f80
staging: comedi: con't rely on comedidev.h to include headers

Removed most the includes from comedidev.h and pushed only the needed
ones to all the comedi drivers.

This one was missed.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>