- x86: move early_ioremap prototypes to io.h

now that the early-ioremap code is unified, move the prototypes too from
io_32.h to io.h.

this fixes:

arch/x86/kernel/setup.c:531: error: implicit declaration of function ‘early_ioremap_init'

on 64-bit.

Signed-off-by: Ingo Molnar <mingo@elte.hu>