src: include primary header first

This is a common practice in C projects, which simply enforces that
each header must compile cleanly without implicit dependencies on
other headers (see also the previous commit).

src/common/dir.c

@@ -4,13 +4,13 @@
  *
  * Copyright Johan Malm 2020
  */
+#include "common/dir.h"
 #include <assert.h>
 #include <glib.h>
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <sys/stat.h>
-#include "common/dir.h"
 #include "common/buf.h"
 #include "common/list.h"
 #include "common/mem.h"