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

folly/detail/FingerprintPolynomial.h

@@ -125,7 +125,7 @@ class FingerprintPolynomial {
     }
   }
 
-  static int size() {
+  static constexpr int size() {
     return 1 + DEG/64;
   }
  private: