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>

src/egl/wayland/wayland-drm/meson.build

@@ -24,7 +24,7 @@ wayland_drm_protocol_c = custom_target(
   'wayland-drm-protocol.c',
   input : 'wayland-drm.xml',
   output : 'wayland-drm-protocol.c',
-  command : [prog_wl_scanner, 'code', '@INPUT@', '@OUTPUT@'],
+  command : [prog_wl_scanner, wl_scanner_arg, '@INPUT@', '@OUTPUT@'],
 )
 
 wayland_drm_client_protocol_h = custom_target(
@@ -61,7 +61,7 @@ linux_dmabuf_unstable_v1_protocol_c = custom_target(
   'linux-dmabuf-unstable-v1-protocol.c',
   input : wayland_dmabuf_xml,
   output : 'linux-dmabuf-unstable-v1-protocol.c',
-  command : [prog_wl_scanner, 'code', '@INPUT@', '@OUTPUT@'],
+  command : [prog_wl_scanner, wl_scanner_arg, '@INPUT@', '@OUTPUT@'],
 )
 
 linux_dmabuf_unstable_v1_client_protocol_h = custom_target(