powerpc/xive: Drop extern qualifiers from header function prototypes

As reported by ./scripts/checkpatch.pl --strict:

CHECK: extern prototypes should be avoided in .h files

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/157384145834.181768.944827793193636924.stgit@bahia.lan