pro2cmake: Consider _LOADED operations as always processed

To remove wrong unused keys comments from the project file.

Change-Id: Ib52b1738b6136a54f6accb86156b9b2609f331c3
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

util/cmake/pro2cmake.py

@@ -1933,6 +1933,7 @@ def write_ignored_keys(scope: Scope, indent: str) -> str:
     for k in sorted(ignored_keys):
         if k in {
             "_INCLUDED",
+            "_LOADED",
             "TARGET",
             "QMAKE_DOCS",
             "QT_SOURCE_TREE",