Fixed building on Windows with SDL_VIDEO=OFF

Fixes https://github.com/libsdl-org/SDL/issues/6562

src/core/windows/SDL_windows.c

@@ -24,6 +24,7 @@
 
 #include "SDL_windows.h"
 #include "SDL_error.h"
+#include "SDL_system.h"
 
 #include <objbase.h>    /* for CoInitialize/CoUninitialize (Win32 only) */
 #if defined(HAVE_ROAPI_H)
@@ -343,4 +344,54 @@ WIN_RectToRECT(const SDL_Rect *sdlrect, RECT *winrect)
 
 #endif /* defined(__WIN32__) || defined(__WINRT__) || defined(__GDK__) */
 
+
+/*
+ * Public APIs
+ */
+#if !defined(SDL_VIDEO_DRIVER_WINDOWS)
+
+#if defined(__WIN32__) || defined(__GDK__)
+int
+SDL_RegisterApp(const char *name, Uint32 style, void *hInst)
+{
+    (void) name;
+    (void) style;
+    (void) hInst;
+    return 0;
+}
+
+void
+SDL_UnregisterApp(void)
+{
+}
+
+void SDL_SetWindowsMessageHook(SDL_WindowsMessageHook callback, void *userdata)
+{
+}
+#endif /* __WIN32__ || __GDK__ */
+
+#if defined(__WIN32__) || defined(__WINGDK__)
+int
+SDL_Direct3D9GetAdapterIndex(int displayIndex)
+{
+    (void) displayIndex;
+    return 0; /* D3DADAPTER_DEFAULT */
+}
+
+SDL_bool
+SDL_DXGIGetOutputInfo(int displayIndex, int *adapterIndex, int *outputIndex)
+{
+    (void) displayIndex;
+    if (adapterIndex) {
+        *adapterIndex = -1;
+    }
+    if (outputIndex) {
+        *outputIndex = -1;
+    }
+    return SDL_FALSE;
+}
+#endif /* __WIN32__ || __WINGDK__ */
+
+#endif /* !SDL_VIDEO_DRIVER_WINDOWS */
+
 /* vi: set ts=4 sw=4 expandtab: */

src/haptic/windows/SDL_dinputhaptic.c

@@ -37,7 +37,11 @@
 /*
  * External stuff.
  */
+#ifdef SDL_VIDEO_DRIVER_WINDOWS
 extern HWND SDL_HelperWindow;
+#else
+static const HWND SDL_HelperWindow = NULL;
+#endif
 
 
 /*

src/joystick/windows/SDL_dinputjoystick.c

@@ -42,7 +42,11 @@
 #define CONVERT_MAGNITUDE(x)    (((x)*10000) / 0x7FFF)
 
 /* external variables referenced. */
+#ifdef SDL_VIDEO_DRIVER_WINDOWS
 extern HWND SDL_HelperWindow;
+#else
+static const HWND SDL_HelperWindow = NULL;
+#endif
 
 /* local variables */
 static SDL_bool coinitialized = SDL_FALSE;