Fix f-o mode not sourcing postsnd when soundings are on (NOAA-EMC#1790)

Soundings are allowed in forecast-only mode, but the config file would
not be sourced during experiment creation. Now the config file is
sourced if bufr soundings (`DO_BUFRSND`) are on.

Resolves NOAA-EMC#1431

workflow/applications/gfs_forecast_only.py

@@ -29,6 +29,9 @@ def _get_app_configs(self):
         if self.do_atm and self.do_metp:
             configs += ['metp']
 
+        if self.do_bufrsnd:
+            configs += ['postsnd']
+
         if self.do_gempak:
             configs += ['gempak']