Do not distribute _gen/param-spec-struct.h in tarballs or scan it with g-i

It isn't self-contained, and provokes a (non-fatal) syntax error from
g-ir-scanner.

Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Acked-by: Philip Withnall
Bug: https://bugzilla.gnome.org/show_bug.cgi?id=697261