lua: Fix multilib buildpath reproducibility issues

The .pc we install ourselves for lua has hardcoded /lib assumptions in it
which means in a multilib environment, full build paths end up in users
like rpm's configuration.

Fix the .pc file to use a correct includedir and libdir to resolve
those reproducibility issues.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>