xen: arm: fix build of hvm.c

xen: arm: fix build of hvm.c

Add include of xsm/xsm.h to fix:

hvm.c: In function 'do_hvm_op': hvm.c:37:9: error: implicit declaration of function 'xsm_hvm_param' [-Werror=implicit-function-declaration]
hvm.c:37:9: error: nested extern declaration of 'xsm_hvm_param' [-Werror=nested-externs]
hvm.c:37:28: error: 'XSM_TARGET' undeclared (first use in this function)
hvm.c:37:28: note: each undeclared identifier is reported only once for each function it appears in
cc1: all warnings being treated as errors

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>