mruby-dir: add missing unistd.h include for posix systems

mkdtemp() requires unistd.h on posix systems like macos and linux.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-dir/test/dirtest.c

@@ -13,6 +13,8 @@
 #if defined(_WIN32)
 #include <io.h>
 #include <direct.h>
+#else
+#include <unistd.h>
 #endif
 
 static void