AP_GPS: avoid use of mavlink_channel_t when GCS not compiled in

[peterbarker]

Board               AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
Durandal                       0      *           0       0                 0      0      0
HerePro             0                                                                     
Hitec-Airspeed      *                                                                     
KakuteH7-bdshot                0      *           0       0                 0      0      0
MatekF405                      0      *           0       0                 0      0      0
Pixhawk1-1M-bdshot             0                  0       0                 0      0      0
f103-QiotekPeriph   *                                                                     
f303-Universal      -72                                                                   
iomcu                                                           *                         
revo-mini                      0      *           0       0                 0      0      0
skyviper-v2450                                    0