Fix method redefinition warnings in threads on Perl >= 5.16

$drh gets cleared on CLONE, but the driver methods persist.
Due to a bug, this didn't warn until Perl 5.16

$ perl -Mthreads -MDBI -MDBD::ODBC -we 'DBI->setup_driver("DBD::ODBC");' \
  -e 'DBD::ODBC->driver; threads->create(sub { DBD::ODBC->driver })->join'
Subroutine DBI::st::odbc_lob_read redefined at ${archlib}/DBI.pm line 1393.
Subroutine DBI::st::odbc_rows redefined at ${archlib}/DBI.pm line 1393.
Subroutine DBI::st::odbc_getdiagrec redefined at ${archlib}/DBI.pm line 1393.
Subroutine DBI::db::odbc_getdiagrec redefined at ${archlib}/DBI.pm line 1393.
Subroutine DBI::db::odbc_getdiagfield redefined at ${archlib}/DBI.pm line 1393.
Subroutine DBI::st::odbc_getdiagfield redefined at ${archlib}/DBI.pm line 1393.
Merge pull request #1 from ilmari/thread-driver-warning

Fix method redefinition warnings in threads on Perl >= 5.16