repo: Introspect _sign_commit() and _verify_commit()

We can't use #ifdef in the headers, since then g-ir-scanner won't pick
up the functions (unless we included config.h).  Let's instead always
have the symbols, but just set an error if we were built without
support for it, just like how pull works.

src/libostree/ostree-repo.h

@@ -466,7 +466,6 @@ gboolean ostree_repo_pull (OstreeRepo             *self,
                            GCancellable           *cancellable,
                            GError                **error);
 
-#ifdef HAVE_GPGME
 gboolean ostree_repo_sign_commit (OstreeRepo     *self,
                                   const gchar    *commit_checksum,
                                   const gchar    *key_id,
@@ -480,7 +479,6 @@ gboolean ostree_repo_verify_commit (OstreeRepo   *self,
                                     GFile        *extra_keyring,
                                     GCancellable *cancellable,
                                     GError      **error);
-#endif
 
 G_END_DECLS