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: cefc7ca ("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>

drivers/acpi/prmt.c

@@ -50,7 +50,7 @@ struct prm_context_buffer {
 #pragma pack()
 
 
-LIST_HEAD(prm_module_list);
+static LIST_HEAD(prm_module_list);
 
 struct prm_handler_info {
 	guid_t guid;