In contrib/uuid-ossp, #include headers needed for ntohl() and ntohs().

Oversight in commit b8cc8f947.  I just noticed this causes compiler
warnings on FreeBSD, and it really ought to cause warnings elsewhere too:
all references I can find say that <arpa/inet.h> is required for these.
We have a lot of code elsewhere that thinks that both <netinet/in.h>
and <arpa/inet.h> should be included for these functions, so do it that
way here too, even though <arpa/inet.h> ought to be sufficient according
to the references I consulted.

Back-patch to 9.4 where the previous commit landed.

contrib/uuid-ossp/uuid-ossp.c

@@ -17,6 +17,10 @@
 #include "utils/builtins.h"
 #include "utils/uuid.h"
 
+/* for ntohl/htonl */
+#include <netinet/in.h>
+#include <arpa/inet.h>
+
 /*
  * It's possible that there's more than one uuid.h header file present.
  * We expect configure to set the HAVE_ symbol for only the one we want.