x11: Fix underlinking when --as-needed is used

Add the x11-xcb module as dependency as well. The corresponding library
is needed for its XGetXCBConnection symbol, which will be reported as
missing when the platform plug-in was built with the --as-needed linker
flag.

(cherry picked from commit 6def870)

platform/x11/CMakeLists.txt

@@ -1,7 +1,7 @@
 # libcogplaform-x11
 
 pkg_check_modules(COGPLATFORM_X11_DEPS IMPORTED_TARGET
-    REQUIRED wpebackend-fdo-1.0>=1.6.0 egl xcb xkbcommon-x11)
+    REQUIRED wpebackend-fdo-1.0>=1.6.0 egl xcb xkbcommon-x11 x11-xcb)
 
 add_library(cogplatform-x11 MODULE cog-platform-x11.c)
 set_target_properties(cogplatform-x11 PROPERTIES