gvfs-gdu-volume-monitor: Define build dependency on libgnome-keyring

Build fails due to missing libgnome-keyring static library
(libgnome-keyring.la). libgnome-keyring has to be
explicitly listed as one of the dependencies.

Signed-off-by: Adam YH Lee <adam.yh.lee@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>