ACPI: PRM: make symbol 'prm_module_list' static

ACPI: PRM: make symbol 'prm_module_list' static

The sparse tool complains as follows:

drivers/acpi/prmt.c:53:1: warning:
 symbol 'prm_module_list' was not declared. Should it be static?

This symbol is not used outside of prmt.c, so marks it static.

Fixes: cefc7ca46235 ("ACPI: PRM: implement OperationRegion handler for the PlatformRtMechanism subtype")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>