building with perfc=y was broken

because of b38d426ad0 ("x86/viridian: flush remote tlbs
by hypercall") which was defining mshv_call_flush, but using
mshv_flush.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>

xen/arch/x86/hvm/viridian.c

@@ -607,7 +607,7 @@ int viridian_hypercall(struct cpu_user_regs *regs)
          * See Microsoft Hypervisor Top Level Spec. sections 12.4.2
          * and 12.4.3.
          */
-        perfc_incr(mshv_flush);
+        perfc_incr(mshv_call_flush);
 
         /* These hypercalls should never use the fast-call convention. */
         status = HV_STATUS_INVALID_PARAMETER;