output: fix timestamp missing usecs

On ARM 32bit with Musl `tv_usecs` is defined as `int64_t` which lead to
CreateIsoTimeString() printing all zeros on the usecs. Work around this
by first assigning to a `int64_t` and then updating the expected format
string to accept `int64_t`.

Bug: #5094.

src/util-time.c

@@ -222,8 +222,9 @@ void CreateIsoTimeString (const struct timeval *ts, char *str, size_t size)
         WinStrftime(ts, t, str, size);
 #else
         char time_fmt[64] = { 0 };
-        strftime(time_fmt, sizeof(time_fmt), "%Y-%m-%dT%H:%M:%S.%%06u%z", t);
-        snprintf(str, size, time_fmt, ts->tv_usec);
+        int64_t usec = ts->tv_usec;
+        strftime(time_fmt, sizeof(time_fmt), "%Y-%m-%dT%H:%M:%S.%%06" PRIi64 "%z", t);
+        snprintf(str, size, time_fmt, usec);
 #endif
     } else {
         snprintf(str, size, "ts-error");