esp_now: fix unused variable warning
esp_now: fix unused variable warning

esp_now_pkt_hdr_t *hdr is only used if gnrc_sixlowpan is used,
so move it into the according #ifdef

fixes #12257

kYc0o/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,ant9000/RIOT,mtausig/RIOT,authmillenon/RIOT,x3ro/RIOT,kYc0o/RIOT,josephnoir/RIOT,ant9000/RIOT,smlng/RIOT,kaspar030/RIOT,toonst/RIOT,yogo1212/RIOT,smlng/RIOT,rfuentess/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,toonst/RIOT,jasonatran/RIOT,basilfx/RIOT,rfuentess/RIOT,aeneby/RIOT,yogo1212/RIOT,rfuentess/RIOT,ant9000/RIOT,josephnoir/RIOT,OTAkeys/RIOT,rfuentess/RIOT,miri64/RIOT,mtausig/RIOT,kaspar030/RIOT,josephnoir/RIOT,mtausig/RIOT,basilfx/RIOT,josephnoir/RIOT,jasonatran/RIOT,aeneby/RIOT,toonst/RIOT,mtausig/RIOT,authmillenon/RIOT,authmillenon/RIOT,mtausig/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kYc0o/RIOT,authmillenon/RIOT,miri64/RIOT,rfuentess/RIOT,basilfx/RIOT,smlng/RIOT,yogo1212/RIOT,miri64/RIOT,x3ro/RIOT,toonst/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,jasonatran/RIOT,ant9000/RIOT,yogo1212/RIOT,aeneby/RIOT,miri64/RIOT,aeneby/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,jasonatran/RIOT,kaspar030/RIOT,x3ro/RIOT,RIOT-OS/RIOT,x3ro/RIOT,josephnoir/RIOT,OlegHahm/RIOT,kaspar030/RIOT,basilfx/RIOT,OlegHahm/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,ant9000/RIOT,miri64/RIOT,basilfx/RIOT,jasonatran/RIOT,smlng/RIOT,authmillenon/RIOT,OlegHahm/RIOT,smlng/RIOT,toonst/RIOT,x3ro/RIOT,aeneby/RIOT,yogo1212/RIOT