BUG/MINOR: logs: leave startup-logs global and not per-thread

Commit f8188c6 ("MEDIUM: threads/logs: Make logs thread-safe") made logs
thread-local but it also made the copy of the startup-logs thread-local,
meaning that when threads are configured, upon startup the list of startup
logs appears to be empty. Let's just remove the THEAD_LOCAL directive
there, as the check for the startup period is already present.

This fix should be backported to 1.8.

src/log.c

@@ -250,7 +250,7 @@ THREAD_LOCAL char *logline_rfc5424 = NULL;
 
 /* A global buffer used to store all startup alerts/warnings. It will then be
  * retrieve on the CLI. */
-static THREAD_LOCAL char *startup_logs = NULL;
+static char *startup_logs = NULL;
 
 struct logformat_var_args {
 	char *name;