g-ir-tool-templates.in: Use the correct Python module filename extension on Windows

Hi,

In the preparation for the support (move) of the build system to Meson, tools/g-ir-tools-template.in was updated to look for the installed _giscanner Python module. However, since on Windows the filename extension for Python module is .pyd, not .so, the installed _giscanner.pyd could never be found by the script, causing introspection builds for packages to fail as the _giscanner module could not be found.

This makes the script look for the _giscanner module properly on Windows.

With blessings, thank you!