build: drop the stdarg gnulib module

gnulib fixes a portability problem on AIX which is a platform we have
never targetted.
Reviewed-by: NJán Tomko <jtomko@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

bootstrap.conf

@@ -94,7 +94,6 @@ sigpipe
 snprintf
 socket
 stat-time
-stdarg
 strchrnul
 strdup-posix
 strndup