Use IMPORTED_TARGET for GnomeKeyring.

Summary:
Otherwise we would need to append also GNOME_KEYRING_LIBRARY_DIRS to
link_directories.

/usr/bin/ld: cannot find -lgnome-keyring
c++: error: linker command failed with exit code 1 (use -v to see invocation)
ninja: build stopped: subcommand failed.

Reviewers: Falkon, Automotive, kkofler, drosca

Subscribers: falkon

Tags: Falkon

Differential Revision: https://phabricator.kde.org/D12980