x86/boot: Explicitly include realmode.h to handle RM reservations

Explicitly include asm/realmode.h, which provides reserve_real_mode(),
instead of picking it up by an indirect include of asm/acpi.h.  acpi.h
will soon stop including realmode.h so that changing realmode.h doesn't
require a full kernel rebuild.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Link: https://lkml.kernel.org/r/20191126165417.22423-3-sean.j.christopherson@intel.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/setup.c

@@ -35,6 +35,7 @@
 #include <asm/kaslr.h>
 #include <asm/mce.h>
 #include <asm/mtrr.h>
+#include <asm/realmode.h>
 #include <asm/olpc_ofw.h>
 #include <asm/pci-direct.h>
 #include <asm/prom.h>