lib: Coerce flags enums to GIR bitfields

The GI scanner decides if an `enum` is really a `bitfield` if it finds
any values that have left shifts. With an `enumeration`, the
introspecting language may error or convert to a different type if the
user tries to combine values. Change all Flags `enum`s to use
left-shifted values so that they're represented as `bitfield`s in the
GIR.

The primary bug here is that you can't combine `REFS_ONLY` and
`NO_PRUNE` when calling `OSTree.Repo.prune()` from an introspected
language.

This is an IABI break since the typelib will change from `enumeration`
to `bitfield`. `OstreeRepoImportFlags` is internal but the change is
included here to prepare for a subsequent name that would require bit
shifting to operate correctly as a flag.

src/libostree/ostree-gpg-verify-result.h

@@ -134,7 +134,7 @@ GVariant * ostree_gpg_verify_result_get_all (OstreeGpgVerifyResult *result,
  * for future variations.
  **/
 typedef enum {
-  OSTREE_GPG_SIGNATURE_FORMAT_DEFAULT = 0
+  OSTREE_GPG_SIGNATURE_FORMAT_DEFAULT = (0 << 0),
 } OstreeGpgSignatureFormatFlags;
 
 _OSTREE_PUBLIC