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.
1 file changed