contrib: qt: fix .pc generation for qml_plugins

The qml_plugins did not include a rewrite rule like those that were written in 09e3cc77. Consequently, the .pc files were invalid and linking to the contrib build folder instead of the installation prefix folder. This commit is the same as the previous commit but dedicated to qml_plugins specifically, instead of just qt_plugins.

Please check locally if you still find path to the contrib build folder if you can test the contrib. :)