gui/configure.json: Make -{system,qt}-xcb force enabling xcb

... just like -xcb does implicitly. Otherwise, failure to detect
system-xcb would silently fall back to -no-xcb despite obviously
contradicting the user's request (-qt-xcb always worked anyway,
as there is no test that can fail).

Change-Id: I6f3145fac0881e7847c4a70547fce206e797a9bb
Reviewed-by: Jake Petroules <jake.petroules@qt.io>

src/gui/configure.json

@@ -1157,6 +1157,7 @@
             "label": "XCB",
             "section": "Platform plugins",
             "autoDetect": "!config.darwin",
+            "enable": "input.xcb == 'system' || input.xcb == 'qt'",
             "condition": "libs.xcb",
             "output": [ "privateFeature" ]
         },