sys/net/gnrc_pktbuf_static: fix compilation with -Wcast-align
sys/net/gnrc_pktbuf_static: fix compilation with -Wcast-align

Comparing the pointers as `uintptr_t` won't hurt but makes the compiler
happy.

miri64/RIOT,kYc0o/RIOT,OlegHahm/RIOT,kYc0o/RIOT,miri64/RIOT,kaspar030/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,jasonatran/RIOT,kaspar030/RIOT,miri64/RIOT,kYc0o/RIOT,ant9000/RIOT,jasonatran/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,OlegHahm/RIOT,kaspar030/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,ant9000/RIOT,jasonatran/RIOT,miri64/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,ant9000/RIOT,miri64/RIOT,ant9000/RIOT,ant9000/RIOT,OlegHahm/RIOT