platform/x86/intel/pmc: Remove unneeded extern keyword in header

Remove unneeded extern keyword in header file. Functions are
extern by default so extern keyword is not unnecessary for
function declaration.

Signed-off-by: Xi Pardee <xi.pardee@linux.intel.com>
Link: https://lore.kernel.org/r/20250214214416.10150-4-xi.pardee@linux.intel.com


Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>