[iostreams] patch for building by BBv2

[hamigaki]

Hello,

I'm testing Boost.Iostreams library for 1.34. 
Then I found that the libraries built by BBv2 don't contain some classes.
This problem occurs because the sources of the "lib boost_iostreams" 
don't contain the following files except zlib.cpp and bzip2.cpp: 

    file_descriptor.cpp
    mapped_file.cpp

These files were contained in the sources for 1.33.1 (BBv1). 
So I think that it's a simple mistake. 
The attachment is the patch for this problem. 

Regards,
Takeshi Mouri

[Jonathan Turkanis]

fixed in [37396]