Improve the log in Universal Compaction to include more debug information.

Summary:
Previously, the log for Universal Compaction does not include the current
number of files in case the compaction is triggered by the number of files.
This diff includes the number of files in the log.

Test Plan:
make

db/compaction_picker.cc

@@ -553,8 +553,8 @@ Compaction* UniversalCompactionPicker::PickCompaction(
       if ((c = PickCompactionUniversalReadAmp(
                mutable_cf_options, version, score, UINT_MAX,
                num_files, log_buffer)) != nullptr) {
-        LogToBuffer(log_buffer, "[%s] Universal: compacting for file num\n",
-                    version->cfd_->GetName().c_str());
+        LogToBuffer(log_buffer, "[%s] Universal: compacting for file num -- %u\n",
+                    version->cfd_->GetName().c_str(), num_files);
       }
     }
   }