BUILD: protocol: fix some build errors on OpenBSD

Building 1.6 and above on OpenBSD 5.2 fails due to protocol.c not
including sys/types.h before sys/socket.h :

  In file included from src/protocol.c:14:
  /usr/include/sys/socket.h:162: error: expected specifier-qualifier-list before 'u_int8_t'

This fix needs to be backported to 1.6.
diff --git a/src/protocol.c b/src/protocol.c
index 3caccb6..7884ef7 100644
--- a/src/protocol.c
+++ b/src/protocol.c
@@ -10,6 +10,7 @@
  *
  */
 
+#include <sys/types.h>
 #include <sys/socket.h>
 
 #include <common/config.h>