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/libotutil/otutil.h

@@ -60,6 +60,9 @@
 #include <ot-variant-utils.h>
 #include <ot-variant-builder.h>
 #include <ot-checksum-utils.h>
-#include <ot-gpg-utils.h>
 #include <ot-checksum-instream.h>
 #include <ot-tool-util.h>
+
+#ifndef OSTREE_DISABLE_GPGME
+#include <ot-gpg-utils.h>
+#endif