package/netsurf: make sure host libpng can be found

Commit 42c8f9f6 ("package/netsurf: add
dependency on host-libpng for sdl") ensured host-libpng was built
prior to netsurf when the SDL backend is selected. However, this is
not sufficient for the netsurf build system to find libpng on the
host, we need to help by providing the right HOST_CFLAGS and
HOST_LDFLAGS.

Fixes:

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



Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
[Thomas: rewrite commit message.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>