w32: Fix default registry path

* configure.ac (GNUPG_REGISTRY_DIR): Remove leading backslash.

Windows does not like the leading backslash and won't read
the key.
Problem reported in the Gpg4win Message boards.

This bug was introduced by rev. 75ba215e

• Signed-off-by: Andre Heinecke <aheinecke@intevation.de>