[libthr] revert change of visibility of _thread_keytable to unbreak debugger

Fix regression by r318539. The sysutils/pstack uses library libthread_db to
read information about threads state. The function pt_ta_new makes lookup of
several key symbols including _thread_keytable. But r318539 mades this field
static. It causes silent ignore of libthr library by pstack and as result
sysutils/pstack doesn't output any thread information.

This fix changes this field back to non-static.

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D11738