Add definition of file_descriptor_source::open()

[here-mikelley]

This commit adds the missing definition of the overload of
file_descriptor_source::open() that accepts a boost::filesystem::path.

[mclow]

Tests?

[codecov]

Codecov Report

Merging #114 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #114   +/-   ##
========================================
  Coverage    68.84%   68.84%           
========================================
  Files           80       80           
  Lines         3444     3444           
  Branches      1027     1027           
========================================
  Hits          2371     2371           
  Misses         454      454           
  Partials       619      619           

Impacted Files	Coverage Δ
include/boost/iostreams/device/file_descriptor.hpp	100.00% <ø> (ø)

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