Add missing top level include guards

[anonymous]

This ticket is addressed by this pull request: ​https://github.com/boostorg/function/pull/13/files

The missing top level include guard means that even if boost/function.hpp is included in a pre-compiled header, we still pay the heavy boost pp pre-processing cost every time it is also included.

This is very problematic with Visual Studio intellisense: It pre-process and parses the translation unit on the fly to provide syntax/semantic highlighting.

In our projects, this fix remove a couple of seconds in this process. (boost/function.hpp is included in many headers that result in pre-processing it multiple time per compilation units)