AP_RSSI: replace header guard with pragma once

libraries/AP_RSSI/AP_RSSI.h

@@ -13,9 +13,7 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
- 
-#ifndef AP_RSSI_H
-#define AP_RSSI_H
+#pragma once
 
 #include <AP_HAL/AP_HAL.h>
 #include <AP_Param/AP_Param.h>
@@ -77,5 +75,3 @@ private:
     // Scale and constrain a float rssi value to 0.0 to 1.0 range 
     float scale_and_constrain_float_rssi(float current_rssi_value, float low_rssi_range, float high_rssi_range);
 };
-
-#endif // AP_RSSI_H