[bbr] fix bbr header C compliance (#7272)

Fix compile errors when including backbone_router_ftd.h in C files.

include/openthread/backbone_router_ftd.h

@@ -351,12 +351,12 @@ void otBackboneRouterSetNdProxyCallback(otInstance *                    aInstanc
  * Represents the Backbone Router ND Proxy info.
  *
  */
-struct otBackboneRouterNdProxyInfo
+typedef struct otBackboneRouterNdProxyInfo
 {
     otIp6InterfaceIdentifier *mMeshLocalIid;             ///< Mesh-local IID
     uint32_t                  mTimeSinceLastTransaction; ///< Time since last transaction (Seconds)
     uint16_t                  mRloc16;                   ///< RLOC16
-};
+} otBackboneRouterNdProxyInfo;
 
 /**
  * This method gets the Backbone Router ND Proxy info.

include/openthread/instance.h

@@ -53,7 +53,7 @@ extern "C" {
  * @note This number versions both OpenThread platform and user APIs.
  *
  */
-#define OPENTHREAD_API_VERSION (178)
+#define OPENTHREAD_API_VERSION (179)
 
 /**
  * @addtogroup api-instance