sparc: fix sparse warning in kernel/pmc.c

Fix following warning:
arch/sparc/kernel/pmc.c:41:6: warning: symbol 'pmc_swift_idle'
was not declared. Should it be static?

It was not used outside this file - make it static

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>