Fix compiler warning:
trx/trx0purge.c: In function 'trx_purge_add_update_undo_to_history':
trx/trx0purge.c:254:16: error: variable 'page_header' set but not used [-Werror=unused-but-set-variable]
trx/trx0purge.c:252:15: error: variable 'seg_header' set but not used [-Werror=unused-but-set-variable]