tree-wide: Make sure all headers are standalone

Let's make sure all of our headers include everything they require.

src/basic/pcapng.h

@@ -1,6 +1,8 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include <stdint.h>
+
 /*
  * For details about the file format see RFC:
  *   https://www.ietf.org/id/draft-tuexen-opsawg-pcapng-03.html