ARM: fix bios32.c build warning

arch/arm/kernel/bios32.c: In function 'pcibios_fixup_bus':
arch/arm/kernel/bios32.c:302: warning: unused variable 'root'

caused by 9f786d03 (arm/PCI: get rid of device resource fixups)

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>