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/Core/System.cpp

@@ -32,6 +32,7 @@
 #include "Core/PowerPC/PowerPC.h"
 #include "IOS/USB/Emulated/Infinity.h"
 #include "IOS/USB/Emulated/Skylanders/Skylander.h"
+#include "IOS/USB/USBScanner.h"
 #include "VideoCommon/Assets/CustomAssetLoader.h"
 #include "VideoCommon/CommandProcessor.h"
 #include "VideoCommon/Fifo.h"
@@ -89,6 +90,7 @@ struct System::Impl
   SerialInterface::SerialInterfaceManager m_serial_interface;
   Sram m_sram;
   SystemTimers::SystemTimersManager m_system_timers;
+  IOS::HLE::USBScanner m_usb_scanner;
   VertexShaderManager m_vertex_shader_manager;
   XFStateManager m_xf_state_manager;
   VideoInterface::VideoInterfaceManager m_video_interface;
@@ -313,6 +315,11 @@ SystemTimers::SystemTimersManager& System::GetSystemTimers() const
   return m_impl->m_system_timers;
 }
 
+IOS::HLE::USBScanner& System::GetUSBScanner() const
+{
+  return m_impl->m_usb_scanner;
+}
+
 VertexShaderManager& System::GetVertexShaderManager() const
 {
   return m_impl->m_vertex_shader_manager;