gnu: libblockdev: Set default configuration directory.

Fix error from 'udisks': “Failed to get contents of the config
dir (/etc/libblockdev/conf.d/)Error opening directory
"/etc/libblockdev/conf.d/": No such file or directory.”

* gnu/packages/disk.scm (libblockdev)[arguments]: Add phase
'patch-configuration-directory'.

Signed-off-by: Efraim Flashner <efraim@flashner.co.il>