qt5/qt5webchannel: add qwebchannel.js installation in /var/www

To use webchannel in an application qwebchannel.js is needed but this file was not
installed.

Signed-off-by: Julien Corjon <corjon.j@ecagroup.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>