Mark FingerprintPolynomial.size() constexpr
Mark FingerprintPolynomial.size() constexpr

Summary: Because it is, and so that MSVC doesn't complain about things in the fingerprint table generator.

Reviewed By: yfeldblum

Differential Revision: D4178562

fbshipit-source-id: 8816f413bc100b42dd72f0e991aec8559b328e9b

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