gobject-introspection: bump require gobject-introspection version

see: https://bugzilla.gnome.org/show_bug.cgi?id=709462
https://git.gnome.org/browse/gobject-introspection/commit/?id=f6770c808203d4d3eb9a30e5c4bc896f990cae34
Merge pull request #221 from cosmo0920/fix-signal-flags-gi-1_38_0

gobject-introspection: raise require gi version to run SignalFlags assertion

Patch by cosmo. Thanks!!!

Suggested by mtasaka. Thanks!!!