Add outline of complex class from MP

Author: mborland

include/boost/decimal.hpp

@@ -37,6 +37,7 @@
 #include <boost/decimal/type_traits.hpp>
 #include <boost/decimal/detail/io.hpp>
 #include <boost/decimal/cstdio.hpp>
+#include <boost/decimal/complex.hpp>
 
 #if defined(__clang__) && !defined(__GNUC__)
 #  pragma clang diagnostic pop