Fix a warning from using a variable called "truncate":
../src/btree/bt_curnext.c: In function '__wt_btcur_next':
../src/btree/bt_curnext.c:374:43: error: declaration of 'truncate' shadows a global declaration [-Werror=shadow]
../src/btree/bt_curprev.c: In function '__wt_btcur_prev':
../src/btree/bt_curprev.c:466:43: error: declaration of 'truncate' shadows a global declaration [-Werror=shadow]