Compile warning in function BoatCitaTxInit

[PanHH]

Describe the bug
Compile warning in function BoatCitaTxInit

api_cita.c: In function 'BoatCitaTxInit':
api_cita.c:111:14: warning: unused variable 'retval_str' [-Wunused-variable]
     BCHAR   *retval_str = NULL;
              ^~~~~~~~~~

[Cavalierliu]

problem fixed