vendor/zlib: always include <errno.h> in gzguts.h

ober

38ab4bfa319b45f74cb23eddf798ad1897f6aa78

diff --git a/vendor/ChezScheme/zlib/gzguts.h b/vendor/ChezScheme/zlib/gzguts.h
index 266305d..bc08e4e 100644
--- a/vendor/ChezScheme/zlib/gzguts.h
+++ b/vendor/ChezScheme/zlib/gzguts.h
@@ -128,8 +128,8 @@
 #  include <windows.h>
 #  define zstrerror() gz_strwinerror((DWORD)GetLastError())
 #else
+#  include <errno.h>             /* gzread.c uses errno/EAGAIN/EWOULDBLOCK unconditionally */
 #  ifndef NO_STRERROR
-#    include <errno.h>
 #    define zstrerror() strerror(errno)
 #  else
 #    define zstrerror() "stdio error (consult errno)"