libid3tag: Convert license format to SPDX format.

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

libs/libid3tag/Makefile

@@ -16,7 +16,7 @@ PKG_SOURCE_URL:=@SF/mad
 PKG_MD5SUM:=e5808ad997ba32c498803822078748c3
 PKG_MAINTAINER:=Ted Hess <thess@kitschensync.net>
 
-PKG_LICENSE:=GPLv2
+PKG_LICENSE:=GPL-2
 PKG_LICENSE_FILES:=COPYING
 
 PKG_FIXUP:=autoreconf