Remove a pair of unnecessary dependencies on datapath-protocol.h.

These headers don't really need datapath-protocol.h.  connmgr.h indirectly
used "struct nlattr" from that header, so add a forward declaration.  (The
next commit will remove use of struct nlattr entirely from that header,
since it is not really appropriate.)

lib/flow.h

@@ -24,7 +24,6 @@
 #include "openflow/nicira-ext.h"
 #include "openflow/openflow.h"
 #include "hash.h"
-#include "openvswitch/datapath-protocol.h"
 #include "util.h"
 
 struct dpif_flow_stats;

lib/netdev-linux.h

@@ -19,7 +19,6 @@
 
 #include <stdint.h>
 #include <stdbool.h>
-#include "openvswitch/datapath-protocol.h"
 
 /* These functions are Linux specific, so they should be used directly only by
  * Linux-specific code. */

ofproto/connmgr.h

@@ -23,6 +23,7 @@
 #include "openflow/nicira-ext.h"
 #include "openvswitch/types.h"
 
+struct nlattr;
 struct ofconn;
 struct ofopgroup;
 struct ofputil_flow_removed;