[border-router] add missing otBorderRoutingDhcp6PdGetState() declaration (#9654)

This commit adds `otBorderRoutingDhcp6PdGetState()` function
declaration in `border_routing.h` which was already implemented in
`border_routing_api.cpp` file.

include/openthread/border_routing.h

@@ -444,6 +444,18 @@ otError otBorderRoutingGetNextRouterEntry(otInstance                         *aI
  */
 void otBorderRoutingDhcp6PdSetEnabled(otInstance *aInstance, bool aEnabled);
 
+/**
+ * Gets the current state of DHCPv6 Prefix Delegation.
+ *
+ * Requires `OPENTHREAD_CONFIG_BORDER_ROUTING_DHCP6_PD_ENABLE` to be enabled.
+ *
+ * @param[in]  aInstance  A pointer to an OpenThread instance.
+ *
+ * @returns The current state of DHCPv6 Prefix Delegation.
+ *
+ */
+otBorderRoutingDhcp6PdState otBorderRoutingDhcp6PdGetState(otInstance *aInstance);
+
 /**
  * @}
  *

include/openthread/instance.h

@@ -53,7 +53,7 @@ extern "C" {
  * @note This number versions both OpenThread platform and user APIs.
  *
  */
-#define OPENTHREAD_API_VERSION (377)
+#define OPENTHREAD_API_VERSION (378)
 
 /**
  * @addtogroup api-instance