Fix missing file causing turret errors

gamemodes/zombiesurvival/gamemode/sh_channel.lua

@@ -0,0 +1,27 @@
+GM.MaxChannels = {}
+GM.MaxChannels["prop_gunturret"] = 7
+
+function GM:GetFreeChannel(class)
+	local max = self.MaxChannels[class]
+	if not max then return 1 end
+
+	local taken_channels = {}
+
+	for _, ent in pairs(ents.FindByClass(class)) do
+		if ent:IsValid() and ent.GetChannel then
+			taken_channels[ent:GetChannel()] = true
+		end
+	end
+
+	for i=1, max do
+		if not taken_channels[i] then
+			return i
+		end
+	end
+
+	return -1
+end
+
+function GM:HasFreeChannel(class)
+	return self:GetFreeChannel(class) >= 1
+end