LibGUI: Add missing LibCore/File include

This was previously included via ConfigFile.h

Userland/Libraries/LibGUI/TextEditor.cpp

@@ -10,6 +10,7 @@
 #include <AK/ScopeGuard.h>
 #include <AK/StringBuilder.h>
 #include <AK/TemporaryChange.h>
+#include <LibCore/File.h>
 #include <LibCore/Timer.h>
 #include <LibGUI/Action.h>
 #include <LibGUI/AutocompleteProvider.h>