AP_Mount do not use flexible array in union

We actually don't want a flexible array in this union, but rather a way
to access it byte by byte. This fixes the build for gcc >= 6

In file included from ../../libraries/AP_GPS/AP_GPS_UBLOX.cpp:23:0:
../../libraries/AP_GPS/AP_GPS_UBLOX.h:387:23: error: flexible array member in union
         uint8_t bytes[];
                       ^
compilation terminated due to -Wfatal-errors.

In file included from ../../libraries/AP_Mount/AP_Mount.cpp:9:0:
../../libraries/AP_Mount/AP_Mount_Alexmos.h:291:23: error: flexible array member in union
         uint8_t bytes[];
                       ^
compilation terminated due to -Wfatal-errors.

libraries/AP_Mount/AP_Mount_Alexmos.h

@@ -288,7 +288,7 @@ private:
         alexmos_angles angles;
         alexmos_params params;
         alexmos_angles_speed angle_speed;
-        uint8_t bytes[];
+        uint8_t bytes[1];
     } _buffer,_current_parameters;
 
     AP_HAL::UARTDriver *_port;

libraries/AP_Mount/AP_Mount_SToRM32_serial.h

@@ -145,7 +145,7 @@ private:
     union PACKED SToRM32_reply {
         SToRM32_reply_data_struct data;
         SToRM32_reply_ack_struct ack;
-        uint8_t bytes[];
+        uint8_t bytes[1];
     } _buffer;
 
     // keep the last _current_angle values