AP_HAL: fix warning due to missing prototype after HAL rework

This was introduced with the HAL rework:

In file included from /p/ardupilot/libraries/AP_HAL/AP_HAL.h:11:0,
                 from /p/ardupilot/ArduCopter/Copter.h:35,
                 from /p/ardupilot/ArduCopter/ArduCopter.cpp:76:
/p/ardupilot/ArduCopter/ArduCopter.cpp: In function 'int ArduPilot_main(int, char* const*)':
/p/ardupilot/libraries/AP_HAL/AP_HAL_Main.h:11:26: warning: no previous declaration for 'int ArduPilot_main(int, char* const*)' [-Wmissing-declarations]
 #define AP_MAIN __EXPORT ArduPilot_main
                          ^

It's due to PX4 using that warning as opposed to Linux. Since it harmless, add
the prototype for everybody.

libraries/AP_HAL/AP_HAL_Main.h

@@ -18,6 +18,7 @@
 #if CONFIG_MAIN_WITHOUT_ARGC_ARGV
 
 #define AP_HAL_MAIN() extern "C" { \
+    int AP_MAIN(void); \
     int AP_MAIN(void) { \
         AP_HAL::HAL::FunCallbacks callbacks(setup, loop); \
         hal.run(0, NULL, &callbacks); \
@@ -26,6 +27,7 @@
     }
 
 #define AP_HAL_MAIN_CALLBACKS(CALLBACKS) extern "C" { \
+    int AP_MAIN(void); \
     int AP_MAIN(void) { \
         hal.run(0, NULL, CALLBACKS); \
         return 0; \
@@ -35,6 +37,7 @@
 #else
 
 #define AP_HAL_MAIN() extern "C" { \
+    int AP_MAIN(int argc, char* const argv[]); \
     int AP_MAIN(int argc, char* const argv[]) { \
         AP_HAL::HAL::FunCallbacks callbacks(setup, loop); \
         hal.run(argc, argv, &callbacks); \
@@ -43,6 +46,7 @@
     }
 
 #define AP_HAL_MAIN_CALLBACKS(CALLBACKS) extern "C" { \
+    int AP_MAIN(int argc, char* const argv[]); \
     int AP_MAIN(int argc, char* const argv[]) { \
         hal.run(argc, argv, CALLBACKS); \
         return 0; \