Fixes compilation errors when using project as rebar dependency.

Created by: idubrov

Please review the fix that removes explicit include directories configuration in rebar.config and uses include_lib to include pkt.hrl.

Without the fix gen_icmp could not be used as rebar dependency, since rebar will download all deps (including gen_icmp deps) into /deps and gen_icmp without the patch expects dependency to be available at /deps/gen_icmp/deps