headers MAINTENANCE headers/include cleanup

- add missing extern "C" in some of the public headers for use in C++
- includes cleanup and fix ordering
diff --git a/tests/utests/test_hash_table.c b/tests/utests/test_hash_table.c
index f705db0..3e17552 100644
--- a/tests/utests/test_hash_table.c
+++ b/tests/utests/test_hash_table.c
@@ -24,6 +24,7 @@
 #include <cmocka.h>
 
 #include <string.h>
+#include <stdlib.h>
 #include <stdio.h>
 
 #include "../../src/hash_table.h"