message: build: Clean remnants of fix
  A fix for was introduced in (v0.11.0, Qt 5.2.1).
  was fixed in version 5.3.0.
  The separated patch file, provided by was dropped in while
  bumping Qt to 5.9.4 (5.9.6). But libxcb.mk remained unchanged.
  This change reverts for libxcb.mk.
diff: @@ -27,13 +27,8 @@ define $(package)_preprocess_cmds
  sed "s/pthread-stubs//" -i configure
  endef
  -# Don't install xcb headers to the default path in order to work around a qt
  -# build issue: https://bugreports.qt.io/browse/QTBUG-34748
  -# When using qt's internal libxcb, it may end up finding the real headers in
  -# depends staging. Use a non-default path to avoid that.
  -
  define $(package)_config_cmds
  - $($(package)_autoconf) --includedir=$(host_prefix)/include/xcb-shared
  + $($(package)_autoconf)
  endef
  define $(package)_build_cmds
  