misc/mei: Allow building standalone for compile testing

misc/mei: Allow building standalone for compile testing

While this is not a particularly useful configuration, the MEI code should
compile even when no drivers for a GPU containing a management engine are
built.

Cc: Usyskin, Alexander <alexander.usyskin@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Simon Richter <Simon.Richter@hogyros.de>
Link: https://patch.msgid.link/20260107182615.488194-4-Simon.Richter@hogyros.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>