Fix sign conversion in idb main
Fix sign conversion in idb main

Summary: `dispatch_source_create` takes an unsigned int

Reviewed By: c-ryan747

Differential Revision: D24884143

fbshipit-source-id: 50b722c8e19a366a51db6d5753359683c74ef355

facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl