add core-deps/libportal.bst

We can no longer use the version from freedesktop-sdk as it now depends on gtk4.

We need to coordinate the merge within all modules at once: eog!115 (merged) epiphany!1051 (merged) gnome-builder!486 (merged) nautilus!749 (merged)

P.S: this depends on !1417 (merged) being merged first.