ffmpeg: convert license to SPDX format.

Signed-off-by: Ian Leonard <antonlacon@gmail.com>

multimedia/ffmpeg/Makefile

@@ -16,7 +16,7 @@ PKG_SOURCE_URL:=http://ffmpeg.org/releases/
 PKG_MD5SUM:=ea25fc252c132141f12988ed5506bfe8
 PKG_MAINTAINER:=Ted Hess <thess@kitschensync.net>
 
-PKG_LICENSE:=LGPLv2.1+ GPLv2+ LGPLv3
+PKG_LICENSE:=LGPL-2.1+ GPL-2+ LGPL-3
 PKG_LICENSE_FILES:=COPYING.GPLv2 COPYING.GPLv3 COPYING.LGPLv2.1 COPYING.LGPLv3
 
 FFMPEG_CUSTOM_ENCODERS:= \