lib: heap: Use proper flexible array

0 length array is a GNU extension. Use proper C99 flexible
array.

Signed-off-by: Flavio Ceolin <[email protected]>

Author: ceolin

lib/heap/heap.h

@@ -74,7 +74,7 @@ struct z_heap {
 	size_t allocated_bytes;
 	size_t max_allocated_bytes;
 #endif
-	struct z_heap_bucket buckets[0];
+	struct z_heap_bucket buckets[];
 };
 
 static inline bool big_heap_chunks(chunksz_t chunks)