gnu: libreoffice: Update to 5.3.7.2 and enable GTK3.

* gnu/packages/libreoffice.scm (libreoffice): Update to 5.3.7.2
[source]: Change URL to HTTPS.
[inputs]: Add gtk+.
[configure-flags]: Remove "--disable-gtk3".

Signed-off-by: Marius Bakke <mbakke@fastmail.com>