[hyperdrive] add unistd.h include for read()/write()/close()

The _POSIX_C_SOURCE define alone is insufficient when a precompiled
header includes stdio.h before gzguts.h - the feature test macro has
no effect after headers are already processed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: falken42

gzguts.h

@@ -49,6 +49,8 @@
 #if defined(__TURBOC__) || defined(_MSC_VER) || defined(_WIN32)
 #  include <io.h>
 #  include <sys/stat.h>
+#else
+#  include <unistd.h>       /* [hyperdrive]: needed for read()/write()/close() */
 #endif
 
 #if defined(_WIN32) && !defined(WIDECHAR)