package/freerdp: also install to staging

freerdp installs a library that other packages may use, so
we must also install it to staging.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>