Xos.h: include <strings.h> on FreeBSD for index/rindex

Adding index() and rindex() as macros wreaks havoc with several libc++
headers, which liberally use the name 'index' for variables, functions,
and more. Get the declarations from <strings.h> instead, where they have
been for at least two decades.

Signed-off-by: Dimitry Andric <dimitry@andric.com>