AP_Scripting: include extern-hal line to fix compilation

this external line was coming in from a header file somewhere...

libraries/AP_Scripting/generator/src/main.c

@@ -2924,6 +2924,8 @@ int main(int argc, char **argv) {
   // for set_and_print_new_error_message deprecate warning
   fprintf(source, "#include <AP_Scripting/lua_scripts.h>\n");
 
+  fprintf(source, "extern const AP_HAL::HAL& hal;\n");
+
   trace(TRACE_GENERAL, "Starting emission");
 
   emit_headers(source);