meson: stop exporting internal wayland details

meson: stop exporting internal wayland details

With version v1.15 the "code" option was deprecated in favour of
"private-code" or "public-code".

Before the interface symbol generated was exported (which is a bad idea
since it's internal implementation detail) and others may misuse it.

That was the case with libva approx. 1 year ago. Since then libva was
fixed, so we can finally hide it by using "private-code"

Inspired by similar xserver patch by Adam Jackson.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>