s_scalbn.c: Add missing float.h include

This caused LDBL_MANT_DIG to not be defined and therefore the scalbnl
alias was not being emitted for double==long double platforms.

Fixes: 760b2ffc ("Update scalbn* functions to the musl versions")
Reported by: Jenkins

(cherry picked from commit f5542795b99206a2b4e5a57429d18b9478264e24)