AP_GPS: 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.cpp:24:0:
../../libraries/AP_GPS/AP_GPS_ERB.h:93:23: error: flexible array member in union
         uint8_t bytes[];
                       ^
compilation terminated due to -Wfatal-errors.

In file included from ../../libraries/AP_GPS/AP_GPS_ERB.cpp:22:0:
../../libraries/AP_GPS/AP_GPS_ERB.h:93:23: error: flexible array member in union
         uint8_t bytes[];
                       ^
compilation terminated due to -Wfatal-errors.

In file included from ../../libraries/AP_GPS/AP_GPS_MTK.cpp:25:0:
../../libraries/AP_GPS/AP_GPS_MTK.h:75:23: error: flexible array member in union
         uint8_t bytes[];
                       ^
compilation terminated due to -Wfatal-errors.

In file included from ../../libraries/AP_GPS/AP_GPS_MTK19.cpp:26:0:
../../libraries/AP_GPS/AP_GPS_MTK.h:75:23: error: flexible array member in union
         uint8_t bytes[];
                       ^
compilation terminated due to -Wfatal-errors.

In file included from ../../libraries/AP_GPS/AP_GPS_SIRF.cpp:22:0:
../../libraries/AP_GPS/AP_GPS_SIRF.h:101:23: error: flexible array member in union
         uint8_t bytes[];
                       ^
compilation terminated due to -Wfatal-errors.

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.

libraries/AP_GPS/AP_GPS_ERB.h

@@ -90,7 +90,7 @@ private:
         erb_stat stat;
         erb_dops dops;
         erb_vel vel;
-        uint8_t bytes[];
+        uint8_t bytes[1];
     } _buffer;
 
     enum erb_protocol_bytes {

libraries/AP_GPS/AP_GPS_MTK.h

@@ -72,7 +72,7 @@ private:
     // Receive buffer
     union PACKED {
         diyd_mtk_msg msg;
-        uint8_t bytes[];
+        uint8_t bytes[1];
     } _buffer;
 
     // Buffer parse & GPS state update

libraries/AP_GPS/AP_GPS_MTK19.h

@@ -78,6 +78,6 @@ private:
     // Receive buffer
     union {
         diyd_mtk_msg msg;
-        uint8_t bytes[];
+        uint8_t bytes[1];
     } _buffer;
 };

libraries/AP_GPS/AP_GPS_SIRF.h

@@ -98,7 +98,7 @@ private:
     // Message buffer
     union {
         sirf_geonav nav;
-        uint8_t bytes[];
+        uint8_t bytes[1];
     } _buffer;
 
     bool        _parse_gps(void);

libraries/AP_GPS/AP_GPS_UBLOX.h

@@ -384,7 +384,7 @@ private:
         ubx_rxm_rawx rxm_rawx;
 #endif
         ubx_ack_ack ack;
-        uint8_t bytes[];
+        uint8_t bytes[1];
     } _buffer;
 
     enum ubs_protocol_bytes {