Require Znumtheory before using it (#548)

For rocq-prover/stdlib#136

Author: andres-erbsen

lib/IEEE754_extra.v

@@ -19,6 +19,7 @@
     floating-point numbers, on top of the Flocq library. *)
 
 From Coq Require Import Reals SpecFloat ZArith Psatz Bool Eqdep_dec.
+From Coq Require Znumtheory.
 From Flocq Require Import Core Digits Operations Round Bracket Sterbenz
                           BinarySingleNaN Binary Round_odd.