zlib: fix include of zlib.h

Including <zlib.h> may lead to false positives when the user specifies a bad
path in `./configure --shared-zlib --shared-zlib-includes=/path/to/zlib`. If
a zlib.h exists somewhere on the system include path (common on UNIX systems),
the compiler will include that instead, possibly leading to header mismatch
errors that are hard to debug.

src/node_zlib.cc

@@ -25,8 +25,8 @@
 #include <string.h>
 #include <stdlib.h>
 #include <sys/types.h>
-#include <zlib.h>
 
+#include "zlib.h"
 #include "node.h"
 #include "node_buffer.h"