package/gobject-introspection: fix datarootdir

datarootdir has been removed since version 1.66 and
https://gitlab.gnome.org/GNOME/gobject-introspection/-/commit/a88b1ac548516ab25140a2e9dea8cac894bbac93
resulting in the following build failures with autotools-based
applications using introspection (e.g. libmediaart or libsecret) if
gobject-introspection is not installed on host since commit
abc110e3:

make[3]: *** No rule to make target 'MediaArt-2.0.typelib', needed by 'all'.  Stop.

or

Makefile:2917: /usr/share/gobject-introspection-1.0/Makefile.introspection: No such file or directory

Fixes:
 - http://autobuild.buildroot.org/results/70585b81fd6aa78e2aa6e5173f35c0f26a1408a9
 - http://autobuild.buildroot.org/results/95aa8e189f3326ecae6133e298c56ffcbbc3198f



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>