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/libostree.sym

@@ -340,6 +340,7 @@ global:
 
 LIBOSTREE_2016.6 {
 global:
-        ostree_repo_remote_fetch_summary_with_options;
+        ostree_gpg_verify_result_require_valid_signature;
         ostree_raw_file_to_archive_z2_stream;
+        ostree_repo_remote_fetch_summary_with_options;
 } LIBOSTREE_2016.5;