Moves misplaced var from airlock_control.dm

Moved explosion_resistance from airlock_control.dm to airlock.dm proper.

code/game/machinery/doors/airlock.dm

@@ -25,6 +25,7 @@
 	icon_state = "door_closed"
 	power_channel = ENVIRON
 
+	explosion_resistance = 15
 	var/aiControlDisabled = 0 //If 1, AI control is disabled until the AI hacks back in and disables the lock. If 2, the AI has bypassed the lock. If -1, the control is enabled but the AI had bypassed it earlier, so if it is disabled again the AI would have no trouble getting back in.
 	var/hackProof = 0 // if 1, this door can't be hacked by the AI
 	var/secondsMainPowerLost = 0 //The number of seconds until power is restored.

code/game/machinery/doors/airlock_control.dm

@@ -6,7 +6,6 @@ obj/machinery/door/airlock
 	var/frequency
 	var/shockedby = list()
 	var/datum/radio_frequency/radio_connection
-	explosion_resistance = 15
 
 obj/machinery/door/airlock/proc/can_radio()
 	if( !arePowerSystemsOn() || (stat & NOPOWER) || isWireCut(AIRLOCK_WIRE_AI_CONTROL) )