cython: swap prefix-mapping patch for an upstream commit

cython: swap prefix-mapping patch for an upstream commit

Upstream has solved the absolute path problem differently by taking
paths relative to the top of the module.  This appears to solve the
problem, at least I've not found any cases where it breaks.

Drop my patch, and backport the relevant commit from upstream.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>