powerpc/perf: Declare static identifier a such

There are three symbols (two variables and a function) that are being used
solely in the same file (imc-pmu.c), thus, these symbols should be static,
but they are not. This was detected by sparse:

	arch/powerpc/perf/imc-pmu.c:31:20: warning: symbol 'nest_imc_refc' was not declared. Should it be static?
	arch/powerpc/perf/imc-pmu.c:37:20: warning: symbol 'core_imc_refc' was not declared. Should it be static?
	arch/powerpc/perf/imc-pmu.c:46:16: warning: symbol 'imc_event_to_pmu' was not declared. Should it be static?

This patch simply adds the 'static' storage-class definition to these
symbols, thus, restricting their usage only in the imc-pmu.c file.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>