ARM: 8923/1: mm: include <asm/vga.h> for vga_base

iomap.c needs <asm/vga.h> for the definition vga_base
to avoid the following warning:

arch/arm/mm/iomap.c:13:15: warning: symbol 'vga_base' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>