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