package/udisks: install to staging

UDisks2 provides a Library API [1] for accessing the UDisks2 service
via "libudisks2.so". For development of UDisks2 clients, install to
staging as well!

[1] http://storaged.org/doc/udisks2-api/2.9.4/ref-library.html



Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>
Reviewed-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>