Define QT.phonon4qt5.module = phonon4qt5 for Qt5-5.12.1

Summary:
I think [1] broke the qt5_phonon.pri file [2] -- the library is no
longer added to the linker arguments.

[1] https://github.com/qt/qtbase/commit/e0926ca4260953a70cdb2a6f84da35517ead1349#diff-4b36b0980a641d6a8d8e640f89c45625
[2] http://package22.nyi.freebsd.org/data/112amd64-default-PR235620PR235622/2019-02-09_20h17m10s/logs/auralquiz-1.0.0_2.log

Reviewers: FreeBSD, sitter, O4 phonon

Reviewed By: sitter, O4 phonon

Differential Revision: https://phabricator.kde.org/D18919