AP_UAVCAN: fix redefinition of UAVCAN_NODE_POOL_BLOCK_SIZE warning

libraries/AP_UAVCAN/examples/UAVCAN_sniffer/UAVCAN_sniffer.cpp

@@ -41,6 +41,9 @@ void loop();
 const AP_HAL::HAL& hal = AP_HAL::get_HAL();
 
 #define UAVCAN_NODE_POOL_SIZE 8192
+#ifdef UAVCAN_NODE_POOL_BLOCK_SIZE
+#undef UAVCAN_NODE_POOL_BLOCK_SIZE
+#endif
 #define UAVCAN_NODE_POOL_BLOCK_SIZE 256
 
 #define debug_uavcan(fmt, args...) do { hal.console->printf(fmt, ##args); } while (0)