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

Merged Dimitry Andric requested to merge dim/xorgproto:fix-index-define-freebsd-1 into master

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.

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