meson: set correct libnautilus-extension version in .pc

Currently, when building with Meson, the version of the pkg-config file
for libnautilus-extension is set to the version of the library itself.
This is incompatible with how it was before - the version was set to the
version of Nautilus. That breaks some modules that build extensions
(e.g. file-roller).

https://bugzilla.gnome.org/show_bug.cgi?id=779472