pro2cmake: Fix QT_BUILD_TREE variable mapping

CMake doesn't have a PROJECT_BUILD_DIR variable, but it does have a
PROJECT_BINARY_DIR variable.

Note that this might still be the wrong thing to do according to
35a30c7ebbd50e8b5b1fad7c00bd6c36b0dca8b9 , but it's still somwehat
better.

Change-Id: I493f82a791d933a16011d91774aaac4bcaffc5e5
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
Reviewed-by: Qt CMake Build Bot

util/cmake/pro2cmake.py

@@ -797,7 +797,7 @@ class Scope(object):
         if operations is None:
             operations = {
                 "QT_SOURCE_TREE": [SetOperation(["${QT_SOURCE_TREE}"])],
-                "QT_BUILD_TREE": [SetOperation(["${PROJECT_BUILD_DIR}"])],
+                "QT_BUILD_TREE": [SetOperation(["${PROJECT_BINARY_DIR}"])],
             }
 
         self._operations = copy.deepcopy(operations)