xen/arm: Include xen/vmap.h in mm.c

As reported by ECLAIR scan, MISRA requires declaration to be visible
(R8.4). This is not the case for ioremap().

Fixes: 2cd02c27d3 ("arm/mpu: Implement stubs for ioremap_attr on MPU")
Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/arm/mm.c

@@ -12,6 +12,7 @@
 #include <xen/grant_table.h>
 #include <xen/guest_access.h>
 #include <xen/mm.h>
+#include <xen/vmap.h>
 
 #include <xsm/xsm.h>