Add includes for windows' getenv/putenv/free

src/engine/client/backend_sdl.cpp

@@ -7,6 +7,7 @@
 #endif
 
 #include <base/detect.h>
+#include <stdlib.h>
 #include "SDL.h"
 #include "SDL_syswm.h"
 #if defined(__ANDROID__)

src/engine/client/input.cpp

@@ -1,6 +1,7 @@
 /* (c) Magnus Auvinen. See licence.txt in the root of the distribution for more information. */
 /* If you are missing that file, acquire a complete release at teeworlds.com.                */
 #include "SDL.h"
+#include <stdlib.h>
 
 #include <base/system.h>
 #include <engine/shared/config.h>