InputCommon: Use nested namespace specifiers where applicable

Source/Core/InputCommon/ControllerInterface/XInput/XInput.h

@@ -18,9 +18,7 @@
 #error You are building this module against the wrong version of DirectX. You probably need to remove DXSDK_DIR from your include path and/or _WIN32_WINNT is wrong.
 #endif
 
-namespace ciface
-{
-namespace XInput
+namespace ciface::XInput
 {
 void Init();
 void PopulateDevices();
@@ -105,5 +103,4 @@ private:
   const BYTE m_subtype;
   const u8 m_index;
 };
-}  // namespace XInput
-}  // namespace ciface
+}  // namespace ciface::XInput