librepo: upgrade 1.14.5 -> 1.15.1

Changelog:
===========
 Adds API support for waiting on network in an event driven manner (new API function lr_handle_network_wait)
 OpenPGP API extension and fixes (new API functions lr_gpg_*)
 Update license format to "LGPL-2.1-or-later"

Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>