Add missing includes

This patch allows the header to be built standalone, as part of clang C++ modules builds.

include/boost/algorithm/string/detail/case_conv.hpp

@@ -15,6 +15,9 @@
 #include <locale>
 #include <functional>
 
+#include <boost/iterator/transform_iterator.hpp>
+#include <boost/range/begin.hpp>
+#include <boost/range/end.hpp>
 #include <boost/type_traits/make_unsigned.hpp>
 
 namespace boost {