buildtools-extended-tarball: add libgomp-dev

This is needed in particular for newer versions of rpm
which would otherwise fail to build due to absence of omp.h header.

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>