repo: Factor out the check of gpg result to a separate function

I plan to add a function for verifying any data which may return the
error about lack of trusted signatures, so let's avoid the redundancy
and put the check in the separate function.

Closes: #310
Approved by: cgwalters

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

@@ -133,4 +133,8 @@ void ostree_gpg_verify_result_describe_variant (GVariant *variant,
                                                 const gchar *line_prefix,
                                                 OstreeGpgSignatureFormatFlags flags);
 
+_OSTREE_PUBLIC
+gboolean ostree_gpg_verify_result_require_valid_signature (OstreeGpgVerifyResult *result,
+                                                           GError **error);
+
 G_END_DECLS