comphelper: fix an IWYU warning

Change-Id: I565ce46c01edfe16d5639cc62cb5a4ca996db349
Reviewed-on: https://gerrit.libreoffice.org/55098Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>

include/IwyuFilter_include.yaml

+---
+assumeFilename: desktop/source/app/app.cxx

include/comphelper/anytostring.hxx

@@ -21,9 +21,10 @@
 #define INCLUDED_COMPHELPER_ANYTOSTRING_HXX
 
 #include <rtl/ustring.hxx>
-#include <com/sun/star/uno/Any.hxx>
 #include <comphelper/comphelperdllapi.h>
 
+namespace com { namespace sun { namespace star { namespace uno { class Any; } } } }
+
 namespace comphelper
 {