fix iteration over a feature's outputs

use precalculated path instead of incorrectly assembling it from
scratch. it accidentally worked when the features happened to be in the
right order, as the iteration variable 'feature' from the calling
function was inherited. however, if the feature was accessed via
dependency resolution, things blew up.

amends 90eee08b3e, which presumably came to be this way due to a missing
adjustment to a refactoring.

Change-Id: I78b0acc0682cfc27a458df014ce14262a65c6241
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

mkspecs/features/qt_configure.prf

@@ -1182,8 +1182,8 @@ defineTest(qtConfCheckFeature) {
     $${fpfx}.available = $$result
     export($${fpfx}.available)
 
-    for (i, $${currentConfig}.features.$${feature}.output._KEYS_): \
-        qtConfProcessOneOutput($$feature, $$i)
+    for (i, $${fpfx}.output._KEYS_): \
+        qtConfProcessOneOutput($${1}, $$i)
 
     return(true)
 }