vfs_io_uring: hide a possible definition of struct open_how in liburing/compat.h

vfs_io_uring: hide a possible definition of struct open_how in liburing/compat.h

liburing.h will include liburing/compat.h, which either includes
linux/openat2.h or defines struct open_how itself.

This will help with the following changes, which will provide
openat2() via libreplace's system/filesys.h, either including
linux/openat2.h or defining open_how ourself.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>