common,w32: Sync read_w32_registry_string with the gpgrt version

* common/w32-reg.c (get_root_key): Add short version of the root
classes.

The code here is only used by gpgconf's new --show-configs command.
Usually on Windows the code from gpgrt is used for reading the
registry. This one here is an exception and when backporting it I
missed to add the HKCU etc al string.

• Fixes-commit: 6c6c404883e52545ed38293384c95fdacb7227c4
• GnuPG-bug-id: T5724