package/qt5/qt5declarative: fix parallel install

Installing qt5declarative examples on fast/fast/multicore machines sometimes
failes with a variation of the following error messages:

 - Cannot touch [...]/chapter5-listproperties/app.qml: No such file or directory
 - Error copying [...]/chapter2-methods/app.qml: Destination file exists

Fix it by using OTHER_FILES instead of a seperate qml files install target
to fix the race between install_target, install_qml and install_sources.

Fixes:

 - https://gitlab.com/buildroot.org/buildroot/-/jobs/565470221



Signed-off-by: Romain Naour <romain.naour@gmail.com>
[Reworked patch and commit log]
Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>