[mle] restrict SetPreferredRouterId() API to REF_DEVICE (#12973)

This commit restricts the API to set a preferred router ID under
`OPENTHREAD_CONFIG_REFERENCE_DEVICE_ENABLE`. This feature is intended
for testing and therefore be excluded from standard builds to ensure
compliance with the Thread Specification.

src/core/thread/mle_ftd.cpp

@@ -3296,6 +3296,7 @@ exit:
     return;
 }
 
+#if OPENTHREAD_CONFIG_REFERENCE_DEVICE_ENABLE
 Error Mle::SetPreferredRouterId(uint8_t aRouterId)
 {
     Error error = kErrorNone;
@@ -3307,6 +3308,7 @@ Error Mle::SetPreferredRouterId(uint8_t aRouterId)
 exit:
     return error;
 }
+#endif
 
 void Mle::SetRouterId(uint8_t aRouterId)
 {