package/netsurf: fix build of host tool convert_image

package/netsurf: fix build of host tool convert_image

Upstream commit
http://source.netsurf-browser.org/netsurf.git/commit/frontends/framebuffer/Makefile?id=40cdf498b9153685b4cefe576e59f6b41090912b

renamed the variable HOST_*FLAGS to BUILD_*FLAGS.

This breaks the build of the host tool convert_image when png.h is not
provided by the host system itself.

Fixes:
http://autobuild.buildroot.net/results/6ac/6ac71c7a41dd92a5fb924af9da7cedc40e745cfa/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>