gobject-introspection: fix default search path for girdir

When running g-ir-scanner, we get the following error:

  Couldn't find include 'GObject-2.0.gir' (search path:
  '['/usr/lib64', 'gir-1.0', '/usr/local/share/gir-1.0', '/usr/share/gir-1.0',
  '/usr/share/gir-1.0', '/usr/share/gir-1.0']')

This is because g-ir-tool-template.in is not setting girdir correctly.
It's using the prefix instead of the actual girdir.

Note that we don't get such error at do_rootfs time because the extra
directories are speicified. But we will get such error at runtime when
multilib is enabled.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>