x86: Fix parse_reservetop() build failure on certain configs

Commit e67a807f ("x86: Fix 'reservetop=' functionality") added a
fixup_early_ioremap() call to parse_reservetop() and declared it
in io.h.

But asm/io.h was only included indirectly - and on some configs
not at all, causing a build failure on those configs.

Cc: Liang Li <liang.li@windriver.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Wang Chen <wangchen@cn.fujitsu.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
LKML-Reference: <1272621711-8683-1-git-send-email-liang.li@windriver.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>