gpg: conditionally build GPG-related code for sign/verification

Do not build the code related to GPG sign and verification if
GPGME support is disabled.
Public functions return error 'G_IO_ERROR_NOT_SUPPORTED' in case if
gpg-related check is rquested.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>

Closes: #1889
Approved by: cgwalters

src/libostree/ostree-core.c

@@ -2462,6 +2462,7 @@ _ostree_compare_timestamps (const char   *current_rev,
 }
 
 
+#ifndef OSTREE_DISABLE_GPGME
 GVariant *
 _ostree_detached_metadata_append_gpg_sig (GVariant   *existing_metadata,
                                           GBytes     *signature_bytes)
@@ -2487,6 +2488,7 @@ _ostree_detached_metadata_append_gpg_sig (GVariant   *existing_metadata,
 
   return g_variant_dict_end (&metadata_dict);
 }
+#endif /* OSTREE_DISABLE_GPGME */
 
 /**
  * _ostree_get_default_sysroot_path: