package/gobject-introspection: bump to version 1.68.0

Other changes:
  - Change -Dgi_cross_use_host_gi=true to -Dgi_cross_use_prebuilt_gi=true as the
    option has changed.
  - Explicitly define python3 in both the host and target builds of
    g-ir-tool-template.in

Signed-off-by: Adam Duskett <aduskett@gmail.com>
[yann.morin.1998@free.fr:
  - add hash for giscanner/scannerlexer.l used as license file
  - two spaces in hash file
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>