tpm: xen-tpmfront: fix missing declaration of xen_domain

xen-tpmfront fails to build on arm64 with the following error:

drivers/char/tpm/xen-tpmfront.c: In function ‘xen_tpmfront_init’:
drivers/char/tpm/xen-tpmfront.c:422:2: error: implicit declaration of function ‘xen_domain’ [-Werror=implicit-function-declaration]

Add include of xen/xen.h to fix this.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Ashley Lai <adlai@linux.vnet.ibm.com>
Acked-by: Ashley Lai <adlai@linux.vnet.ibm.com>
Cc: Leonidas Da Silva Barbosa <leosilva@linux.vnet.ibm.com>
Cc: Rajiv Andrade <mail@srajiv.net>
Cc: Marcel Selhorst <tpmdd@selhorst.net>
Cc: Sirrix AG <tpmdd@sirrix.com>
Cc: tpmdd-devel@lists.sourceforge.net