libperf: Add missing 'struct perf_cpu_map' forward declaration to perf/cpumap.h

The perf/cpumap.h header is getting the 'struct perf_cpu_map' forward
declaration by luck, add it.

Acked-by: Ian Rogers <irogers@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>