powerpc/pseries: Declare the acquire/release drc index routines

Add declarations for dlpar_{acquire,release}_drc(...)

They are already marked non-static but were missing a prototype/

[BenH: Added extern to be consistent with the rest of the file]

Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>