Fixing gridfs_file_set_xxx wouldn’t export to module map.

When using clang to export modules, functions set of
mongo_gridfs_file_set_xxx couldn’t export to destination simply because
only the getting methods were marked with BSON_API. Typical scenario is
a third party exporting, such as c library exporting for Swift
language.
Script test-libmongoc, which includes GridFS, passed.
This issue was found by Kyle Jessup and Rockford Wei,
perfect.org