windeployqt not working correctly using a subdirs project

The windeployqt.prf spec does not work correctly when using the "subdirs" template, as it won’t create the "windeployqt" target for the nested Makefiles.

This should be fixed probably by checking for contains(TEMPLATE, subdirs) in the script, as it is e.g. done in the equivalent androiddeployqt spec.

Steps to reproduce:

1. Create a project using the "subdirs" template with two or more subprojects
2. Add CONFIG += windeployqt
3. Run qmake followed by jom windeployqt

This will result in an "Error: Target windeployqt doesn't exist." for the subprojects.