WinRT: build fix for SDL_assert.c

src/SDL_assert.c

@@ -37,8 +37,10 @@
 #else  /* fprintf, _exit(), etc. */
 #include <stdio.h>
 #include <stdlib.h>
+#if ! defined(__WINRT__)
 #include <unistd.h>
 #endif
+#endif
 
 static SDL_assert_state
 SDL_PromptAssertion(const SDL_assert_data *data, void *userdata);