Added stubs for simple Steam Controller support

src/joystick/steam/SDL_steamcontroller.h

@@ -0,0 +1,33 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2017 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+#include "../../SDL_internal.h"
+
+typedef SDL_bool (*SteamControllerConnectedCallback_t)(const char *name, SDL_JoystickGUID guid, int *device_instance);
+typedef void (*SteamControllerDisconnectedCallback_t)(int device_instance);
+
+void SDL_InitSteamControllers(SteamControllerConnectedCallback_t connectedCallback,
+                              SteamControllerDisconnectedCallback_t disconnectedCallback);
+void SDL_GetSteamControllerInputs(int *nbuttons, int *naxes, int *nhats);
+void SDL_UpdateSteamControllers();
+void SDL_UpdateSteamController(SDL_Joystick *joystick);
+void SDL_QuitSteamControllers();
+
+/* vi: set ts=4 sw=4 expandtab: */