Make go generate run whenever middleware.cfg is changed (#530)

Makefile

@@ -10,7 +10,7 @@ all: coredns
 # Phony this to ensure we always build the binary.
 # TODO: Add .go file dependencies.
 .PHONY: coredns
-coredns: deps
+coredns: deps core/zmiddleware.go core/dnsserver/zdirectives.go
 	go build $(BUILD_VERBOSE) -ldflags="-s -w"
 
 .PHONY: docker
@@ -48,6 +48,9 @@ clean:
 	go clean
 	rm -f coredns
 
+core/zmiddleware.go core/dnsserver/zdirectives.go: middleware.cfg
+	go generate coredns.go
+
 .PHONY: gen
 gen:
 	go generate coredns.go