usbcamera: Fix compiler warnings

Change-Id: Ifc9fec5e90f5295d2b4a5b4c8648510565b9ba17
diff --git a/modules/usbcamera/HotplugThread.cpp b/modules/usbcamera/HotplugThread.cpp
index 6c65086..02e7167 100644
--- a/modules/usbcamera/HotplugThread.cpp
+++ b/modules/usbcamera/HotplugThread.cpp
@@ -39,6 +39,7 @@
 }
 
 bool HotplugThread::threadLoop() {
+    (void)mModule;  // silence warning about unused member.
 
     /**
      * Check camera connection status change, if connected, do below: