PCI: fix VGA arbiter header file

Remove reference to vgaarb.c and replace it with a comment about the
arbiter itself.
Reported-by: NTiago Vignatti <tiago.vignatti@nokia.com>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>

include/linux/vgaarb.h

 /*
- * vgaarb.c
+ * The VGA aribiter manages VGA space routing and VGA resource decode to
+ * allow multiple VGA devices to be used in a system in a safe way.
  *
  * (C) Copyright 2005 Benjamin Herrenschmidt <benh@kernel.crashing.org>
  * (C) Copyright 2007 Paulo R. Zanoni <przanoni@gmail.com>