Add SPDX to new file.

Author: BenBrock

include/binsparse/detail/allocator.h

@@ -1,3 +1,9 @@
+/*
+ * SPDX-FileCopyrightText: 2024 Binsparse Developers
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
 #pragma once
 
 #include <stdlib.h>