libroach: initialize range tombstone counter

We forgot to initialize a variable that is used to decide whether a
compaction output file should be marked to undergo another compaction.

Release justification: reduce write-amp by avoiding forcing files
through compactions unnecessarily.

Release note: None

c-deps/libroach/table_props.cc

@@ -130,7 +130,7 @@ class DeleteRangeTblPropCollector : public rocksdb::TablePropertiesCollector {
   }
 
  private:
-  int ntombstones_;
+  int ntombstones_ = 0;
 };
 
 class DeleteRangeTblPropCollectorFactory : public rocksdb::TablePropertiesCollectorFactory {