sdfs: Fix missing NULL option list tremination

Option list for volume_options in sdfs was not NULL
terminated. This resulted in a crash when running in
lcov based builds.

This is rectified by this patch.

fixes: bz#1608566

Change-Id: I5d8730f1ae963ed6adf21d970e4921c5d5d92f62
Signed-off-by: ShyamsundarR <srangana@redhat.com>

xlators/features/sdfs/src/sdfs.c

@@ -1520,4 +1520,5 @@ struct volume_options options[] = {
           .tags = {"sdfs"},
           .description = "Enable/Disable dentry serialize functionality"
         },
+        { .key  = {NULL} },
 };