databases/mysql91-client: Fix PLIST when SASLCLIENT is disabled

> Checking for pkg-plist issues (check-plist)

> Parsing plist

> Checking for items in STAGEDIR missing from pkg-plist

> Checking for items in pkg-plist which are not in STAGEDIR

Error: Missing: lib/mysql/plugin/mysql_native_password.so

> Error: Plist issues found.

Approved by: portmgr (blanket)