LinuxKPI: add UUID_STRING_LEN and GUID_INIT to uuid.h

Add a definition for UUID_STRING_LEN to uuid.h as needed by a driver.
Also add GUID_INIT for drm-kmod [1].

Submitted by: wulf [1]
MFC after: 3 days
Reviewed by: hselasky (earlier), wulf
Differential Revision: https://reviews.freebsd.org/D34235