libsa/rarp.c: Change printf format string

[mig_semihalf.com]

Summary

Change printf format string to avoid compilation termination when
RARP_DEBUG macro is defined.

Test Plan

In stand/libsa/rarp.c after all #include's add:
#define RARP_DEBUG
temporary directory to hold compilation files:
mkdir -p $HOME/obj
set appropriate environmental variables, e.g. ARCH=arm64, TARGET_ARCH=aarch64, MAKEOBJDIRPREFIX=$HOME/obj.
In stand/libsa:
sudo -E make buildenv
make
compliation ends with error.