ffmpeg: fix license file

Somewhere between 1.2.7 and 2.3.1 (version bump in bf76f4319131a107)
the LICENSE file has been renamed to LICENSE.md, so `make legal-info`
is currently broken. Update to the current file name.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/ffmpeg/ffmpeg.mk

@@ -10,7 +10,7 @@ FFMPEG_SITE = http://ffmpeg.org/releases
 FFMPEG_INSTALL_STAGING = YES
 
 FFMPEG_LICENSE = LGPLv2.1+, libjpeg license
-FFMPEG_LICENSE_FILES = LICENSE COPYING.LGPLv2.1
+FFMPEG_LICENSE_FILES = LICENSE.md COPYING.LGPLv2.1
 ifeq ($(BR2_PACKAGE_FFMPEG_GPL),y)
 FFMPEG_LICENSE += and GPLv2+
 FFMPEG_LICENSE_FILES += COPYING.GPLv2