nano: use GNU alias for source download

Download source using the GNU alias in order to get fault-tolerance.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>

utils/nano/Makefile

 PKG_LICENSE_FILES:=COPYING
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
-PKG_SOURCE_URL:=http://www.nano-editor.org/dist/v2.5
+PKG_SOURCE_URL:=@GNU/nano
 PKG_MD5SUM:=a04d77611422ab4b6a7b489650c7a793
 
 PKG_INSTALL:=1