stdinc: make SDL_stack_free evaluate to `((void)(data))` when alloca is usable.

Fixes #15727.

(cherry picked from commit 67e6f5b7ee488dfc3cf4b33620b220ae72ff845e)

include/SDL3/SDL_stdinc.h

diff --git a/include/SDL3/SDL_stdinc.h b/include/SDL3/SDL_stdinc.h
index 205f9bc..1f09176 100644
--- a/include/SDL3/SDL_stdinc.h
+++ b/include/SDL3/SDL_stdinc.h
@@ -1306,7 +1306,7 @@
 #define SDL_stack_free(data)
 #elif !defined(SDL_DISABLE_ALLOCA)
 #define SDL_stack_alloc(type, count)    (type*)alloca(sizeof(type)*(count))
-#define SDL_stack_free(data)
+#define SDL_stack_free(data)            ((void)(data))
 #else
 #define SDL_stack_alloc(type, count)    (type*)SDL_malloc(sizeof(type)*(count))
 #define SDL_stack_free(data)            SDL_free(data)