Fix build with Boost 1.85.0

[cho-m]

Build was failing with Boost 1.85.0:

src/gource_settings.cpp:836:36: error: use of undeclared identifier 'extension'
            std::string file_ext = extension(p);
                                   ^
1 error generated.

Looking at https://www.boost.org/doc/libs/1_85_0/libs/filesystem/doc/deprecated.html, it appears the extension() was removed with recommendation:

Replace extension(p) with p.extension().string().

I see oldest support Boost in https://github.com/acaudwell/Gource/blob/master/INSTALL#L21 is 1.46.

Taking a look at https://www.boost.org/doc/libs/1_46_0/boost/filesystem/convenience.hpp, as long as code isn't setting BOOST_FILESYSTEM_VERSION == 2, it looks like extension() was just an alias, i.e.

    inline std::string extension(const path & p)
    {
      return p.extension().string();
    }

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Seen while updating Boost in Homebrew - https://github.com/Homebrew/homebrew-core/pull/169237