ACPI / EC: Mark the function acpi_ec_add_debugfs() as static in ec_sys.c

Mark the function acpi_ec_add_debugfs() as static in
ec_sys.c because it is not used outside this file.

This eliminates the following warning in ec_sys.c:
drivers/acpi/ec_sys.c:108:5: warning: no previous prototype for ‘acpi_ec_add_debugfs’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>