cffi: remove #include of mem.h
cffi: remove #include of mem.h

zstd.h no longer depends on it after backporting a change to
zstd.h.
terrelln/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard