[nd6] add CONFIG_BORDER_ROUTING_ENABLE guard check (#10759)

This commit adds `CONFIG_BORDER_ROUTING_ENABLE` guard checks to
`nd6.hpp` and `nd6.cpp` source files, as the definitions in these
files are only used when the `BORDER_ROUTING` feature is enabled.

src/core/net/nd6.cpp

@@ -33,6 +33,8 @@
 
 #include "nd6.hpp"
 
+#if OPENTHREAD_CONFIG_BORDER_ROUTING_ENABLE
+
 #include "instance/instance.hpp"
 
 namespace ot {
@@ -323,3 +325,5 @@ NeighborAdvertMessage::NeighborAdvertMessage(void)
 } // namespace Nd
 } // namespace Ip6
 } // namespace ot
+
+#endif // OPENTHREAD_CONFIG_BORDER_ROUTING_ENABLE

src/core/net/nd6.hpp

@@ -38,6 +38,8 @@
 
 #include "openthread-core-config.h"
 
+#if OPENTHREAD_CONFIG_BORDER_ROUTING_ENABLE
+
 #include <stdint.h>
 
 #include <openthread/netdata.h>
@@ -973,4 +975,6 @@ static_assert(sizeof(NeighborAdvertMessage) == 24, "Invalid NeighborAdvertMessag
 } // namespace Ip6
 } // namespace ot
 
+#endif // OPENTHREAD_CONFIG_BORDER_ROUTING_ENABLE
+
 #endif // ND6_HPP_