use gmtime_r() instead of gmtime()

This avoids pulling in gmtime's static buffer:

function                                             old     new   delta
svlogd_main                                         1401    1412     +11
send_headers                                         668     678     +10
gmtime                                                21       -     -21
------------------------------------------------------------------------------
(add/remove: 0/2 grow/shrink: 2/0 up/down: 21/-21)              Total: 0 bytes
   text	   data	    bss	    dec	    hex	filename
 920221	    555	   5804	 926580	  e2374	busybox_old
 920221	    555	   5740	 926516	  e2334	busybox_unstripped
                   ^^^^

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/runit/svlogd.c b/runit/svlogd.c
index 412290c..13de257 100644
--- a/runit/svlogd.c
+++ b/runit/svlogd.c
@@ -347,11 +347,13 @@
 /* NUL terminated */
 static void fmt_time_human_30nul(char *s, char dt_delim)
 {
+	struct tm tm;
 	struct tm *ptm;
 	struct timeval tv;
 
 	gettimeofday(&tv, NULL);
-	ptm = gmtime(&tv.tv_sec);
+	ptm = gmtime_r(&tv.tv_sec, &tm);
+	/* ^^^ using gmtime_r() instead of gmtime() to not use static data */
 	sprintf(s, "%04u-%02u-%02u%c%02u:%02u:%02u.%06u000",
 		(unsigned)(1900 + ptm->tm_year),
 		(unsigned)(ptm->tm_mon + 1),