Minimal update to transform_zstd suggested by iwyu.

This file was not active during the iwyu run, so it will need to be
expanded upon later.

src/transform_zstd.c

@@ -17,12 +17,15 @@
  */
 
 
-#include "n2n.h"
+#include "config.h"  // for HAVE_ZSTD
 
 
 #ifdef HAVE_ZSTD
 
 
+#include "n2n.h"
+
+
 typedef struct transop_zstd {
     // no local data
 } transop_zstd_t;