output/csv: use intermediate time_t var, silence compiler warning

There are platforms where timeval and time_t disagree on the width of
the data type of the field which holds seconds. Passing a pointer to an
unexpected type results in warnings (and probably unreliable execution).

Assign the value which is gotten from a timeval to an intermediate
time_t variable, so that the ctime() invocation becomes portable.

src/output/csv.c

@@ -252,11 +252,13 @@ static GString *gen_header(const struct sr_output *o,
 	/* Some metadata */
 	if (ctx->header && !ctx->did_header) {
 		/* save_gnuplot knows how many lines we print. */
+		time_t secs;
+		secs = hdr->starttime.tv_sec;
 		g_string_append_printf(header,
 			"%s CSV generated by %s %s\n%s from %s on %s",
 			ctx->comment, PACKAGE_NAME,
 			sr_package_version_string_get(), ctx->comment,
-			ctx->title, ctime(&hdr->starttime.tv_sec));
+			ctx->title, ctime(&secs));
 
 		/* Columns / channels */
 		channels = o->sdi ? o->sdi->channels : NULL;