reference: fix offset SD to include elapsed time

This should slow down corrections based on old measurements with large
estimated error in frequency.

reference.c

@@ -985,6 +985,7 @@ REF_SetReference(int stratum,
 
   elapsed = UTI_DiffTimespecsToDouble(&now, ref_time);
   our_offset = offset + elapsed * frequency;
+  offset_sd += elapsed * frequency_sd;
 
   if (!is_offset_ok(our_offset))
     return;