Makefile.am: Build vapi files and use vapi metadata correctly

Regressed from 9019498dfef15efec4d12eee8becc55781062a30
gcr: Add gcr_certificate_get_markup() function

For seahorse to use when implementing its GcrCertificate derived
class.
vala: Remove nullability of GcrCertificate.expiry

valac loses its lunch when this parameter is present.
gck: Add some precondition checks to gck_attributes_find_xxxx()
gcr: Fix typo which prevented headers from installing