Non-UTF-8 encoding in `spf/api/modules/metadata_api.h`

[slyon]

During Debian packaging the following encoding issue was detected by Lintian, please consider converting this file to UTF-8.

W: audioreach-graphservices-dev: national-encoding [usr/include/spf/metadata_api.h]
N:
N:   A file is not valid UTF-8.
N:
N:   Debian has used UTF-8 for many years. Support for national encodings is
N:   being phased out. This file probably appears to users in mangled
N:   characters (also called mojibake).
N:
N:   Packaging control files must be encoded in valid UTF-8.
N:
N:   Please convert the file to UTF-8 using iconv or a similar tool.
N:
N:   Visibility: warning
N:   Show-Always: no
N:   Check: files/encoding
N:   Renamed from: national-encoding-in-text-file
N:   debian-changelog-file-uses-obsolete-national-encoding
N:   debian-control-file-uses-obsolete-national-encoding
N:   debian-copyright-file-uses-obsolete-national-encoding
N:   debian-news-file-uses-obsolete-national-encoding
N:   debian-tests-control-uses-national-encoding
N:   doc-base-file-uses-obsolete-national-encoding
N:   national-encoding-in-debconf-template national-encoding-in-manpage