package/gobject-introspection: fix --includedir

package/gobject-introspection: fix --includedir

Set includedir to $(STAGING_DIR)/usr/share/gir-1.0 instead of . in
Makefile.introspection or g-ir-compiler won't find .gir files resulting
in a build failure for autotools-based based programs such as gconf
since bump of gobject-introspection to version 1.68.0 in commit
abc110e3624aa4e8a9bd8595b3fdc0aec60845e0:

Could not find GIR file 'GObject-2.0.gir'; check XDG_DATA_DIRS or use --includedir
error parsing file GConf-2.0.gir: Failed to parse included gir GObject-2.0

Fixes:
 - http://autobuild.buildroot.org/results/8180d893cbd27512915a7f8a3adb232b93a98ceb

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>