leak-detective: Whitelist thread ID getter

In case an external thread calls into our code and logs messages, a thread
object is allocated that will never be released.  Even if we try to clean
up the object via thread value destructor there is no guarantee that the
thread actually terminates before we check for leaks, which seems to be the
case for the Ada Tasking threads.

src/libstrongswan/utils/leak_detective.c

@@ -494,7 +494,7 @@ static bool register_hooks()
  * List of functions using static allocation buffers or should be suppressed
  * otherwise on leak report.
  */
-char *whitelist[] = {
+static char *whitelist[] = {
 	/* backtraces, including own */
 	"backtrace_create",
 	"strerror_safe",
@@ -604,6 +604,8 @@ char *whitelist[] = {
 	"system__tasking__initialize",
 	"system__tasking__initialization__abort_defer",
 	"system__tasking__stages__create_task",
+	/* in case external threads call into our code */
+	"thread_current_id",
 };
 
 /**