xsm: Drop extern of non-existent variable

xsm: Drop extern of non-existent variable

dummy_xsm_ops was dropped as part of organising XSM to be altcall compatible,
but the extern was accidentally left around.

A later change reintroduced dummy_ops which is logically the same thing, but
is private to xsm/dummy.c

Fixes: 164a0b9653f4 ("xsm: refactor xsm_ops handling")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Daniel P. Smith <dpsmith@apertussolutions.com>