glslang: disable precompiled header

- precompiled headers have reproducible issues [1]

[1] https://autobuilder.yocto.io/pub/repro-fail/oe-reproducible-20210125-3iawhiyz/packages/diff-html



Signed-off-by: Jose Quaresma <quaresma.jose@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>