libowfat: add PKG_LICENSE information

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

libs/libowfat/Makefile

 PKG_SOURCE_URL:=http://dl.fefe.de
 PKG_MD5SUM:=6bbee9a86506419657d87123b7a6f2c1
 PKG_MAINTAINER:=Daniel Golle <daniel@makrotopia.org>
+PKG_LICENSE:=GPL-2.0
+PKG_LICENSE_FILES:=COPYING
 
 include $(INCLUDE_DIR)/package.mk