libsa/arp.c: Change printf format string

[mig_semihalf.com]

Summary

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

Test Plan

In stand/libsa/arp.c after all #include's and #define's add:
#define ARP_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.

[mig_semihalf.com]

libsa/arp.c: Change printf format string

Previous revision was created with reversed diff. This
update introduces correct diff.

[imp]

ah, that suddenly makes more sense.