package/netsurf: add dependency on host-libpng for sdl

libpng is used by the convert_image host tool, which is built when SDL
is enabled.

Fixes:
http://autobuild.buildroot.net/results/d0b/d0bbd563fe5c9463316b2ba0d7ef5553be0563bc/



Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>