clean compile; add missing declaration

[frederich]

There is a compilation problem with MSVC 15.7.1 and /std:c++latest.

1>c:\libraries\boost_1_64_0\boost\typeof\msvc\typeof_impl.hpp(128): error C2988: unrecognizable template declaration/definition
1>c:\libraries\boost_1_64_0\boost\typeof\msvc\typeof_impl.hpp(136): note: see reference to class template instantiation 'boost::type_of::msvc_extract_type<ID,T>' being compiled
1>c:\libraries\boost_1_64_0\boost\typeof\msvc\typeof_impl.hpp(128): error C2143: syntax error: missing ';' before '<'
1>c:\libraries\boost_1_64_0\boost\typeof\msvc\typeof_impl.hpp(128): error C2913: explicit specialization; 'boost::type_of::id2type_impl' is not a specialization of a class template
1>c:\libraries\boost_1_64_0\boost\typeof\msvc\typeof_impl.hpp(128): error C2059: syntax error: '<'
1>c:\libraries\boost_1_64_0\boost\typeof\msvc\typeof_impl.hpp(129): error C2334: unexpected token(s) preceding '{'; skipping apparent function body

The PR fix it.

[frederich]

sf