qt5multimedia: install gsttools library to target

It is built when gst-plugins-base is selected.

Signed-off-by: Fatih Aşıcı <fatih.asici@gmail.com>
Noticed-by: Paweł Gibaszek <gibol666@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>