Fix module build with {fmt} module

Author: mborland

include/boost/decimal/fmt_format.hpp

@@ -25,13 +25,18 @@
 #include <boost/decimal/detail/config.hpp>
 #include <boost/decimal/detail/locale_conversion.hpp>
 #include <boost/decimal/charconv.hpp>
+
+#ifndef BOOST_DECIMAL_BUILD_MODULE
+
 #include <algorithm>
 #include <format>
 #include <iostream>
 #include <string>
 #include <tuple>
 #include <cctype>
 
+#endif // BOOST_DECIMAL_BUILD_MODULE
+
 namespace boost {
 namespace decimal {
 namespace detail {