util-lib: move CONF_DIRS_NULSTR definition to def.h

After all, this is not some compiler or C magic, but something very
specific to how systemd works, hence let's move it into def.h, and out
of macro.h

src/sysctl/sysctl.c

@@ -28,6 +28,7 @@
 #include <string.h>
 
 #include "conf-files.h"
+#include "def.h"
 #include "fd-util.h"
 #include "fileio.h"
 #include "hashmap.h"