Fix [-Wundef] warning in POSIX extension

Author: Girgias

ext/posix/posix.c

@@ -40,10 +40,10 @@
 #include <errno.h>
 #include <grp.h>
 #include <pwd.h>
-#if HAVE_SYS_MKDEV_H
+#ifdef HAVE_SYS_MKDEV_H
 # include <sys/mkdev.h>
 #endif
-#if HAVE_SYS_SYSMACROS_H
+#ifdef HAVE_SYS_SYSMACROS_H
 # include <sys/sysmacros.h>
 #endif