gnome3: prefer gcr-4

Newer distributions are now using gcr-4, while older ones will still have
gcr-3. The APIs used by pinentry have not changed in between versions,
only the top-level header has changed. Also, for gcr-3, only gcr-base-3
is required.

(I have attempted to sign up for a dev.gnupg.org account, but am still
awaiting verification.)

• Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>