Add LICENSE header

Author: yaito3014

include/iris/bits/is_function_object.hpp

@@ -1,6 +1,8 @@
 #ifndef IRIS_BITS_IS_FUNCTION_OBJECT
 #define IRIS_BITS_IS_FUNCTION_OBJECT
 
+// SPDX-License-Identifier: MIT
+
 #include <type_traits>
 
 namespace iris {

include/iris/bits/specialization_of.hpp

@@ -1,6 +1,8 @@
 #ifndef IRIS_BITS_SPECIALIZATION_OF
 #define IRIS_BITS_SPECIALIZATION_OF
 
+// SPDX-License-Identifier: MIT
+
 #include <type_traits>
 
 namespace iris {

include/iris/hash/FNV_hash.hpp

@@ -1,6 +1,8 @@
 #ifndef IRIS_HASH_FNV_HASH_HPP
 #define IRIS_HASH_FNV_HASH_HPP
 
+// SPDX-License-Identifier: MIT
+
 #include <bit>
 #include <memory>
 

include/iris/io_fwd.hpp

@@ -1,6 +1,8 @@
 #ifndef IRIS_IO_FWD_HPP
 #define IRIS_IO_FWD_HPP
 
+// SPDX-License-Identifier: MIT
+
 #include <concepts>
 #include <iosfwd>
 #include <type_traits>