flac: Convert license to SPDX format.

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

libs/flac/Makefile

@@ -18,7 +18,7 @@ PKG_MAINTAINER:=Ted Hess <thess@kitschensync.net>
 
 PKG_INSTALL:=1
 
-PKG_LICENSE:=GFDLv1.2 GPLv2 LGPLv2.1 BSD-3c
+PKG_LICENSE:=GFDL-1.2 GPL-2 LGPL-2.1 BSD-3-Clause
 PKG_LICENSE_FILES:=README COPYING.FDL COPYING.GPL COPYING.LGPL COPYING.Xiph
 
 PKG_FIXUP:=autoreconf