Move SJoystickWin32Control back to the source file to avoid compile problems when _IRR_COMPILE_WITH_DIRECTINPUT_JOYSTICK_ is set. But at least keep distinction between declaration and definition to make it more visible all the functions of that struct are member-functions.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@5007 dfc29bdd-3216-0410-991c-e03cc46cb475