curve25519/internal/field/_asm: go mod tidy to fix x/sys version

[thaJeztah]

Relates to CL 545098, which updated the dependency in the main module.

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Sebastiaan van Stijn:

Patch Set 1:

(1 comment)

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Please don’t reply on this GitHub thread. Visit golang.org/cl/545260.
After addressing review feedback, remember to publish your drafts!