libiio: fix legal info

Fixes:
http://autobuild.buildroot.org/results/778/7789da71553bb633cbea6db83faeee061155b9f8/

The license file is now called COPYING.txt.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libiio/libiio.mk

@@ -8,7 +8,7 @@ LIBIIO_VERSION = 0.6
 LIBIIO_SITE = $(call github,analogdevicesinc,libiio,v$(LIBIIO_VERSION))
 LIBIIO_INSTALL_STAGING = YES
 LIBIIO_LICENSE = LGPLv2.1+
-LIBIIO_LICENSE_FILES = COPYING
+LIBIIO_LICENSE_FILES = COPYING.txt
 
 LIBIIO_CONF_OPTS = -DENABLE_IPV6=ON \
 	-DWITH_LOCAL_BACKEND=$(if $(BR2_PACKAGE_LIBIIO_LOCAL_BACKEND),ON,OFF) \