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-private.h

@@ -190,9 +190,11 @@ _ostree_repo_mode_is_bare (OstreeRepoMode mode)
     mode == OSTREE_REPO_MODE_BARE_USER_ONLY;
 }
 
+#ifndef OSTREE_DISABLE_GPGME
 GVariant *
 _ostree_detached_metadata_append_gpg_sig (GVariant   *existing_metadata,
                                           GBytes     *signature_bytes);
+#endif
 
 GFile *
 _ostree_get_default_sysroot_path (void);