x86, mm: remove unnecessary include file from iomap_32.c

asm/highmem.h inclusion is added to use kmap_atomic_prot_pfn()
by commit bb6d59ca

Now kmap_atomic_prot_pfn is moved to iomap_32.c
by commit dd63fdcc

So the asm/highmem.h inclusion in iomap_32.c is unnecessary now.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
LKML-Reference: <20090315151517.GA29074@localhost.localdomain>
Signed-off-by: Ingo Molnar <mingo@elte.hu>