xen/public: replace typeof() with __typeof__()

xen/public: replace typeof() with __typeof__()

typeof() is available in Xen's build environment, which uses Xen's
compiler.  As these headers are public, they need strict standards
conformance.  Only __typeof__() is officially standardized.

A compiler in standards conformance mode should report:

warning: implicit declaration of function 'typeof' is invalid in C99
[-Wimplicit-function-declaration]

(this has been observed with FreeBSD's kernel build environment)

Based-on-patch-by: Julien Grall <julien@xen.org>, Sun Oct 4 20:33:04 2015 +0100
Signed-off-by: Elliott Mitchell <ehem+xen@m5p.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>