powercap: RAPL: Fix struct declaration in header file

struct rapl_package is declared twice in intel_rapl.h, once at line 80
and once earlier.

Code inspection suggests that the first instance should be struct
rapl_domain rather than rapl_package, so change it.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>