libpng: add license info

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/libpng/libpng.mk

@@ -8,6 +8,8 @@ LIBPNG_VERSION = 1.4.12
 LIBPNG_SERIES = 14
 LIBPNG_SITE = http://$(BR2_SOURCEFORGE_MIRROR).dl.sourceforge.net/project/libpng/libpng$(LIBPNG_SERIES)/$(LIBPNG_VERSION)
 LIBPNG_SOURCE = libpng-$(LIBPNG_VERSION).tar.bz2
+LIBPNG_LICENSE = libpng license
+LIBPNG_LICENSE_FILES = LICENSE
 LIBPNG_INSTALL_STAGING = YES
 LIBPNG_DEPENDENCIES = host-pkg-config zlib