ltp: fix empty ltp-dev package

Currently the headers are not installed and the ltp-dev package is
empty.

This patch adds an include-install make target in the do_install step to
install them in sysroot which ends up as a working ltp-dev package.

Signed-off-by: Jonas Höppner <jonas.hoeppner@garz-fricke.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>