Initialize priority

Fixing following compilation error
```
  CC       zsys.lo                                                                                                                                                                                    
cc1: warnings being treated as errors                                                                                                                                                                 
zsys.c: In function 's_log':                                                                                                                                                                          
zsys.c:1187: error: 'priority' may be used uninitialized in this function                                                                                                                             
make[2]: *** [zsys.lo] Error 1 
```