Fix detection of AT-SPI

Building qtbase 5.9.1 on Linux always results in a build with
accessibility support disabled.

The problem is that the config option for accessibility-atspi-bridge
checks for config.atspi, which isn't defined anywhere - it should
check for libs.atspi (which is set if pkg-config finds atspi-2) instead.

[ChangeLog][QtGui][Platform Specific Changes][Linux/XCB]
Fixed detection of AT-SPI, allowing accessibility support to be built
again.

Task-number: QTBUG-61731
Change-Id: If3bd5dfccda40158c566f8507e34b6877b59b6fb
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@qt.io>
Reviewed-by: Bernhard Rosenkränzer <bero@lindev.ch>

src/gui/configure.json

@@ -434,7 +434,7 @@
     "features": {
         "accessibility-atspi-bridge": {
             "label": "ATSPI Bridge",
-            "condition": "features.accessibility && features.xcb && features.dbus && config.atspi",
+            "condition": "features.accessibility && features.xcb && features.dbus && libs.atspi",
             "output": [ "privateFeature", "feature" ]
         },
         "angle": {