Enable libical-glib API indexes in gtk-doc

[Milan Crha]

	Hello,
while discussing [1] with a gtk-doc maintainer, I recalled that
libical-glib also used to have similar warnings printed when the
documentation had been generated. I tried with the latest git master of
libical and it didn't generate those warnings for me, after which I
checked the related files and realized that it had been disabled to
generate the API indexes in commit 7446557cdaf9d02c138c76916e4f90ef2ab,
unfortunately with no explanation. If it was due to warnings like this
one:
> Warning: multiple "IDs" for constraint linkend: api-index-3.0.
then I attached here a patch which fixes it, as had been suggested
in [1]. There can be some other issues with the API indexes, as
described in [2], but that seems to be related to Fedora or newer
docbook xslt (it's still under investigation), which I'd not bother
with at the moment.

I attached the patch here for libical in a hope to make its
documentation a bit more complete.
	Bye,
	Milan

[1] https://bugzilla.gnome.org/show_bug.cgi?id=784208
[2] https://bugzilla.gnome.org/show_bug.cgi?id=785139
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