x86/boot: Fix memremap() related build failure

x86/boot: Fix memremap() related build failure

The following commit:

  5997efb96756 ("x86/boot: Use memremap() to map the MPF and MPC data")

causes new build failures on certain randconfigs:

  drivers/sfi/sfi_core.c: In function ‘sfi_map_memory’:
  drivers/sfi/sfi_core.c:104:10: error: implicit declaration of function ‘memremap’ [-Werror=implicit-function-declaration]
  drivers/sfi/sfi_core.c:104:31: error: ‘MEMREMAP_WB’ undeclared (first use in this function)

This is a case of parasitic header dependencies: the sfi_core.c file indirectly
includes <linux/io.h> for typical - but not all configs.

Including <linux/io.h> explicitly solves the build failure.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: arnd@arndb.de
Cc: aryabinin@virtuozzo.com
Cc: bp@alien8.de
Cc: bp@suse.de
Cc: brijesh.singh@amd.com
Cc: corbet@lwn.net
Cc: dvyukov@google.com
Cc: dyoung@redhat.com
Cc: glider@google.com
Cc: konrad.wilk@oracle.com
Cc: linux-tip-commits@vger.kernel.org
Cc: luto@kernel.org
Cc: lwoodman@redhat.com
Cc: matt@codeblueprint.co.uk
Cc: mst@redhat.com
Cc: pbonzini@redhat.com
Cc: riel@redhat.com
Cc: rkrcmar@redhat.com
Cc: thomas.lendacky@amd.com
Cc: toshi.kani@hpe.com
Link: http://lkml.kernel.org/r/20170720092307.6xslahuaclmsiffe@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>