Account for realloc moving the memory in gebs_arena_realloc()

example/arena.c

@@ -8,7 +8,7 @@ typedef struct {
 
 int main(void)
 {
-    Gebs_Arena *arena = gebs_arena_make(1024);
+    Gebs_Arena *arena = gebs_arena_make(sizeof(int));
     defer { gebs_arena_destroy(arena); }
 
     Ints ints = {0};