gnu: icecat: Update to 68.7.0-guix0-preview1 [security fixes].

Includes fixes for CVE-2020-6821, CVE-2020-6822, CVE-2020-6825,
CVE-2020-6827, and CVE-2020-6828.

* gnu/packages/gnuzilla.scm (%icecat-version, %icecat-build-id): Update.
(icecat-source): Update gnuzilla commit, base version, and hashes.
* gnu/packages/patches/icecat-makeicecat.patch: Adapt to new version.