ruby: do not parallel install

ruby: do not parallel install

ruby's makefiles are rebuilding several items on every invocation,
even if nothing changed, all of which are factored into libruby.so
and cause it to be relinked as well. This does not seem to affect plain
make invocation, but does cause races in make install:

https://bugzilla.yoctoproject.org/show_bug.cgi?id=14695

Rather than figure out how to use make properly and convince upstream
of it, let's just do serial installs and file a bug:

https://bugs.ruby-lang.org/issues/18506

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>