package/python-argon2-cffi: only enable sse2 when supported

We need to backport a commit so that we can enable/disable sse2 using
the ARGON2_CFFI_USE_SSE2 env variable.

Fixes:
http://autobuild.buildroot.net/results/030/0306d66d081dd0807c577edd50d39075a46d0dd9/build-end.log



Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>