Fix srcdir=builddir build

cmake will create folders which so we need to ignore those
diff --git a/utils/po/CMakeLists.txt b/utils/po/CMakeLists.txt
index d9fca3c..5cd3a90 100644
--- a/utils/po/CMakeLists.txt
+++ b/utils/po/CMakeLists.txt
@@ -1,6 +1,6 @@
 file(GLOB _files "${CMAKE_CURRENT_SOURCE_DIR}/*")
 foreach (_dir ${_files})
-  if (IS_DIRECTORY "${_dir}")
+  if (IS_DIRECTORY "${_dir}" AND EXISTS "${_dir}/CMakeLists.txt")
     add_subdirectory("${_dir}")
     get_filename_component(_lang ${_dir} NAME)
     list(APPEND _langs ${_lang})