Add gcr as an optional depency
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# Description:
<!-- Describe the bug in full detail. -->
Currently `pinentry-gnome3` breaks if `gcr` is not installed and there's no indication that `gcr` is needed instead of `gcr-4` (at least for the gnome3 frontend) in the package page
# Additional info:
* package version(s): 1.3.1-2
* config and/or log files:
* link to upstream bug report, if any:
# Steps to reproduce:
<!-- Describe how to reproduce the bug step by step including the commands -->
1. Install `pinentry` regularly without installing `gcr`
2. Try signing a git commit without setting a different `pinentry-program` in `gpg-agent.conf`
issue