Post-fix patch.

The amount of memory used by for event 'memory/sql/dd::String_type'
is increased with enhancement made in DD framework to add a new column
mysql.columns.is_explicit_collation. This caused main.dd_string test
to fail as we cross the threashold number of allocations. This patch
increases the threshold.

Change-Id: I797aa148fbeb91945e9bce6f34b71dc0bd8e8d2e