intel_rapl: need linux/cpuhotplug.h for enum cpuhp_state

Fixes: 7ebf8eff ("intel_rapl: introduce struct rapl_if_private")
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>