test: Add new headers unit test

test/headers.c

@@ -0,0 +1,25 @@
+/* Check that all libbsd overlayed headers preprocess. */
+#include <sys/cdefs.h>
+#include <sys/endian.h>
+#include <sys/bitstring.h>
+#include <sys/queue.h>
+#include <sys/tree.h>
+#include <sys/poll.h>
+
+#include <err.h>
+#include <getopt.h>
+#include <libutil.h>
+#include <md5.h>
+#include <nlist.h>
+#include <readpassphrase.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <vis.h>
+
+int
+main()
+{
+	return 0;
+}