package/lightning: Fix GNU Mirror Path

Replacing ftpmirror.gnu.org with BR2_GNU_MIRROR variable

Signed-off-by: Brian 'redbeard' Harrington <redbeard@coreos.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/lightning/lightning.mk

@@ -5,7 +5,7 @@
 ################################################################################
 
 LIGHTNING_VERSION = 2.1.0
-LIGHTNING_SITE = http://ftp.gnu.org/gnu/lightning
+LIGHTNING_SITE = $(BR2_GNU_MIRROR)/lightning
 LIGHTNING_LICENSE = LGPLv3+
 LIGHTNING_LICENSE_FILES = COPYING.LESSER
 LIGHTNING_INSTALL_STAGING = YES