powerpc/iseries: Mark signal_vsp_instruction() as maybe unused

signal_vsp_instruction() is currently only used if CONFIG_PROC_FS
is enabled. However logically it has nothing to do with PROC_FS,
so rather than making it depend on that mark it as maybe unused.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>