必要な変数(静的メンバ)を設定します。

.h
public:
enum { JOY1,JOY2,JOYMAX,};

private:
static IDirectInput8* input;

static IDirectInputDevice8* keyboard;
static IDirectInputDevice8* mouse;
static IDirectInputDevice8* joypad[];
static unsigned int joycnt;

.cpp
namespace {
IDirectInput8* GameInput::input;
IDirectInputDevice8* GameInput::keyboard;
IDirectInputDevice8* GameInput::mouse;
IDirectInputDevice8* GameInput::joypad[JOYMAX];
unsigned int GameInput::joycnt;
}