fuzz: put fuzzing stuff in own build tag (#1215)

No need to have this build all the time.

Makefile.fuzz

@@ -25,7 +25,7 @@ echo:
 
 .PHONY: $(PLUGINS)
 $(PLUGINS): echo
-	go-fuzz-build $(REPO)/$(@)
+	go-fuzz-build -tags fuzz $(REPO)/$(@)
 	go-fuzz -bin=./$(@)-fuzz.zip -workdir=fuzz/$(@)
 
 .PHONY: clean

plugin/cache/fuzz.go

+// +build fuzz
+
 package cache
 
 import (

plugin/file/fuzz.go

+// +build fuzz
+
 package file
 
 import (

plugin/proxy/fuzz.go

+// +build fuzz
+
 package proxy
 
 import (

plugin/rewrite/fuzz.go

+// +build fuzz
+
 package rewrite
 
 import (