channels: Add Guile-Git as a dependency of external channels.

Fixes a regression introduced in
aed0a594 whereby external channels would
fail to build due to the lack of a (git) module.

Reported by Alex ter Weele on #guix.

* guix/channels.scm (channel-instance-derivations)[guile-gcrypt]:
Remove.
[dependencies]: New variable.
Use it in the 2nd argument to 'build-channel-instance'.