Fix vapi for BookmarkFile API

This fixes build with clang and stack protector

CC=clang meson -Dc_args="-fstack-protector" build
ninja -C build
build/gitg/gitg

vala@7ad06bca

it is expected on vala 0.43.3

At this time:

$ flatpak run org.gnome.Sdk//master 
[📦 org.gnome.Sdk ~]$ vala --version
Vala 0.43.2.20-e730
[📦 org.gnome.Sdk ~]$ 

which is vala@e7308eab (behind the needed fix)