powerpc/smp: Declare current_set static

current_set extern not needed anymore since
commit eafd825e ("powerpc/64: Simplify __secondary_start
paca->kstack handling")

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/a55eb65c9d7319f0af3c31e3f6ba36522f10003d.1646413435.git.christophe.leroy@csgroup.eu