minijail: install .pc files and scoped_minijail.h

This matches the behaviour of the Chromium OS ebuild for this package:
https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/cd6d6815b1604105cb6b8c40b2e0c3f15f401e3b/chromeos-base/minijail/minijail-10-r38.ebuild#47