pro2cmake: Handle conditions for feature system_webp

This is needed for the qtimageformats build with bundled libwebp.

Change-Id: I4d32392d7362f254e9633197497b6d78e12da559
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

util/cmake/pro2cmake.py

@@ -1694,6 +1694,7 @@ def map_condition(condition: str) -> str:
                     and not feature_name.startswith("system_sqlite")
                     and not feature_name.startswith("system_hunspell")
                     and not feature_name.startswith("system_libb2")
+                    and not feature_name.startswith("system_webp")
                 ):
                     part = "ON"
                 elif feature == "dlopen":