package/sdl2_net: add missing host-pkgconf dependency

package/sdl2_net: add missing host-pkgconf dependency

The sdl2_net configure script uses pkg-config to finx sdl2. If it
doesn't find pkg-config, it tries to locate sdl2-config, and defaults
to /usr/bin/sdl2-config, which causes the build to fail with:

arm-linux-gcc: ERROR: unsafe header/library path used in cross-compilation: '-I/usr/include/SDL2'

Fix this by adding host-pkgconf to the dependencies of sdl2_net. We
could have added the right autoconf cache variable to tell the
configure script where sdl2-config is located, but since pkg-config is
tried first, let's use that.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>