meson: hide C API if Xorg is disabled (like autotools)

When building only Xwayland using Meson some files are always installed.
This causes package conflict if Xwayland is built separately from Xorg.

  include/xorg/compositeext.h
  include/xorg/damage.h
  include/xorg/damagestr.h
  include/xorg/dbestruct.h
  include/xorg/dri3.h
  include/xorg/fb.h
  include/xorg/fboverlay.h
  include/xorg/fbpict.h
  include/xorg/fbrop.h
  include/xorg/geext.h
  include/xorg/geint.h
  include/xorg/glyphstr.h
  include/xorg/mi.h
  include/xorg/micmap.h
  include/xorg/micoord.h
  include/xorg/migc.h
  include/xorg/miline.h
  include/xorg/mioverlay.h
  include/xorg/mipict.h
  include/xorg/mipointer.h
  include/xorg/mipointrst.h
  include/xorg/mistruct.h
  include/xorg/misync.h
  include/xorg/misyncfd.h
  include/xorg/misyncshm.h
  include/xorg/misyncstr.h
  include/xorg/mizerarc.h
  include/xorg/panoramiX.h
  include/xorg/panoramiXsrv.h
  include/xorg/picture.h
  include/xorg/picturestr.h
  include/xorg/present.h
  include/xorg/presentext.h
  include/xorg/randrstr.h
  include/xorg/rrtransform.h
  include/xorg/shadow.h
  include/xorg/shmint.h
  include/xorg/syncsdk.h
  include/xorg/vndserver.h
  include/xorg/wfbrename.h
  include/xorg/xace.h
  include/xorg/xacestr.h
  include/xorg/xorg-server.h
  include/xorg/xvdix.h
  include/xorg/xvmcext.h
  share/aclocal/xorg-server.m4

Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>