AP_MSP: allow baro backends to be individually compiled out

filtering by define name is the most useful bit

libraries/AP_MSP/AP_MSP_Telem_Backend.cpp

@@ -562,7 +562,7 @@ void AP_MSP_Telem_Backend::msp_handle_compass(const MSP::msp_compass_data_messag
 
 void AP_MSP_Telem_Backend::msp_handle_baro(const MSP::msp_baro_data_message_t &pkt)
 {
-#if HAL_MSP_BARO_ENABLED
+#if AP_BARO_MSP_ENABLED
     AP::baro().handle_msp(pkt);
 #endif
 }