Rename POTFILES to POTFILES.in
Is there a reason gstreamer POTFILES are not named as POTFILES.in like gettext suggests (https://www.gnu.org/software/gettext/manual/html_node/po_002fPOTFILES_002ein.html)? I think that's the reason we cannot build the pot file on l10n.gnome.org (https://l10n.gnome.org/module/gstreamer/).