examples/nimble_gatt: fix building w/o DEVELHELP
examples/nimble_gatt: fix building w/o DEVELHELP

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