libotutil: Establish a place for GPG utilities

Add ot-gpg-utils.[ch] and move _ostree_gpg_error_to_gio_error() here.

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

@@ -22,7 +22,7 @@
 
 #include "ostree-gpg-verify-result.h"
 
-#include <gpgme.h>
+#include "otutil.h"
 
 /**
  * OstreeGpgVerifyResult:
@@ -35,8 +35,3 @@ struct OstreeGpgVerifyResult {
   gpgme_ctx_t context;
   gpgme_verify_result_t details;
 };
-
-/* XXX Split these into a separate "GPGME utilities" file? */
-
-void _ostree_gpg_error_to_gio_error (gpgme_error_t gpg_error, GError **error);
-