platform/x86/amd/pmf: Fix compiler warnings in static slider

platform/x86/amd/pmf: Fix compiler warnings in static slider

This patch fixes warnings with -Wmissing-prototypes:

warning: no previous prototype for 'source_as_str' [-Wmissing-prototypes]
warning: no previous prototype for 'slider_as_str' [-Wmissing-prototypes]

Fixes: a82ebb3d800d ("platform/x86/amd/pmf: Add PMF acpi debug support")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202305160220.REQc5T2y-lkp@intel.com/
Suggested-by: Patil Rajesh Reddy <Patil.Reddy@amd.com>
Signed-off-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20230516074531.2885235-1-Shyam-sundar.S-k@amd.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>