Fix uninitialized variable ts_guc_bgw_log_level

In #6188 we introduced a new GUC `timescaledb.bgw_log_level` to control
background workers log level, but if we missed to set the default value
into the global variable leading to an assertion when checking for
default values when defining it in the `_guc_init` process.

Fix it by properly initialize the global variable to the
`log_min_messages`.

src/guc.c

@@ -94,7 +94,7 @@ bool ts_guc_enable_async_append = true;
 bool ts_guc_enable_chunkwise_aggregation = true;
 TSDLLEXPORT bool ts_guc_enable_compression_indexscan = true;
 TSDLLEXPORT bool ts_guc_enable_bulk_decompression = true;
-TSDLLEXPORT int ts_guc_bgw_log_level;
+TSDLLEXPORT int ts_guc_bgw_log_level = log_min_messages;
 TSDLLEXPORT bool ts_guc_enable_skip_scan = true;
 /* default value of ts_guc_max_open_chunks_per_insert and ts_guc_max_cached_chunks_per_hypertable
  * will be set as their respective boot-value when the GUC mechanism starts up */