Fix bad merge of write_iops_log= option for fio 2.1

We need to store the string.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/options.c b/options.c
index d0c53b6..1ca4801 100644
--- a/options.c
+++ b/options.c
@@ -2778,7 +2778,7 @@
 	{
 		.name	= "write_iops_log",
 		.lname	= "Write IOPS log",
-		.type	= FIO_OPT_STR,
+		.type	= FIO_OPT_STR_STORE,
 		.off1	= td_var_offset(iops_log_file),
 		.help	= "Write log of IOPS during run",
 		.category = FIO_OPT_C_LOG,