CPUUsage: adds user guidance when CPU time fails to increase

Author: CarterLi

src/detection/cpuusage/cpuusage.c

@@ -54,6 +54,7 @@ const char* ffGetCpuUsageResult(FFCPUUsageOptions* options, FFlist* result)
                 ffTimeSleep(options->waitTime);
                 goto retry;
             }
+            return "CPU time did not increase. Try increasing wait time.";
         }
     }