sysutils/rdfind: add support for xxhash

In the current form - the support for xxhash depends on whether or not
xxhash is installed in the system where the port is built. See
https://github.com/pauldreik/rdfind#development.

This commit makes (lack of) support for xxhash explicit. If the option
is enabled - the xxhash is added to dependencies and built into rdfind,
if disabled - xxhash support is disabled even if the library is present
in the system.

Reviewed by: dtxdf
Differential Revision: https://reviews.freebsd.org/D58398