IOS: Only have one USBScanner

Some games open two USB interfaces, e.g. /dev/usb/oh0 and /dev/usb/hid.
This was causing us to run two scanning threads at once, using up more
CPU time for scanning than we need to.

Source/Core/DolphinLib.props

@@ -171,6 +171,7 @@
     <ClInclude Include="Common\UPnP.h" />
     <ClInclude Include="Common\VariantUtil.h" />
     <ClInclude Include="Common\Version.h" />
+    <ClInclude Include="Common\WaitableFlag.h" />
     <ClInclude Include="Common\WindowsRegistry.h" />
     <ClInclude Include="Common\WindowSystemInfo.h" />
     <ClInclude Include="Common\WorkQueueThread.h" />