build: Clean remnants of QTBUG-34748 fix

A fix for QTBUG-34748 was introduced in #5915 (v0.11.0, Qt 5.2.1).
QTBUG-34748 was fixed in version 5.3.0.
The separated patch file, provided by #5915, was dropped in #12971 while
bumping Qt to 5.9.4 (5.9.6). But libxcb.mk remained unchanged.

This change reverts #5915 for libxcb.mk.

depends/packages/libxcb.mk

@@ -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