xen: ifdef inclusion of <asm/grant_table.h> in <xen/grant_table.h>

xen: ifdef inclusion of <asm/grant_table.h> in <xen/grant_table.h>

Ifdef-ing inclusion of <asm/grant_table.h> allows to avoid
generation of empty <asm/grant_table.h> for cases when
CONFIG_GRANT_TABLE is not enabled.

The following changes were done for Arm:
<asm/grant_table.h> should be included directly because it contains
gnttab_dom0_frames() macros which is unique for Arm and is used in
arch/arm/domain_build.c.
<asm/grant_table.h> is #ifdef-ed with CONFIG_GRANT_TABLE in
<xen/grant_table.h> so in case of !CONFIG_GRANT_TABLE gnttab_dom0_frames
won't be available for use in arch/arm/domain_build.c.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Acked-by: Julien Grall <jgrall@amazon.com>
Acked-by: Shawn Anastasio <sanastasio@raptorengineering.com>