Fixes runtimes related to mmi_holder and defib_timer (#7237)

Fixes runtime in life.dm,1831: undefined proc or verb /obj/item/organ/internal/mmi_holder/robot/tick defib timer().
Fixes runtime in defib.dm,511: undefined variable /obj/item/organ/internal/mmi_holder/robot/var/defib_timer

code/modules/organs/subtypes/machine.dm

@@ -5,6 +5,7 @@
 	organ_tag = O_CELL
 	parent_organ = BP_TORSO
 	vital = 1
+	var/defib_timer = 1 // This sits in the brain organ slot, but is not a brain.
 
 /obj/item/organ/internal/cell/New()
 	robotize()
@@ -46,6 +47,10 @@
 	sleep(-1)
 	update_from_mmi()
 
+// This sits in the brain organ slot, but is not a brain. Posibrains and dronecores aren't brains either.
+/obj/item/organ/internal/mmi_holder/proc/tick_defib_timer()
+	return
+
 /obj/item/organ/internal/mmi_holder/proc/update_from_mmi()
 
 	if(!stored_mmi.brainmob)