gnrc_icmpv6_error: provide NOPs when compiled without module
gnrc_icmpv6_error: provide NOPs when compiled without module

This is aimed to make the usage code of this module more readable.

kaspar030/RIOT,x3ro/RIOT,gebart/RIOT,miri64/RIOT,cladmi/RIOT,aeneby/RIOT,biboc/RIOT,A-Paul/RIOT,mtausig/RIOT,yogo1212/RIOT,yogo1212/RIOT,OTAkeys/RIOT,authmillenon/RIOT,mfrey/RIOT,ant9000/RIOT,ant9000/RIOT,biboc/RIOT,BytesGalore/RIOT,josephnoir/RIOT,josephnoir/RIOT,rfuentess/RIOT,kYc0o/RIOT,authmillenon/RIOT,OTAkeys/RIOT,cladmi/RIOT,miri64/RIOT,mfrey/RIOT,jasonatran/RIOT,A-Paul/RIOT,biboc/RIOT,x3ro/RIOT,aeneby/RIOT,ant9000/RIOT,biboc/RIOT,smlng/RIOT,jasonatran/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,rfuentess/RIOT,A-Paul/RIOT,toonst/RIOT,cladmi/RIOT,smlng/RIOT,kaspar030/RIOT,kYc0o/RIOT,kYc0o/RIOT,yogo1212/RIOT,gebart/RIOT,josephnoir/RIOT,A-Paul/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,mfrey/RIOT,lazytech-org/RIOT,rfuentess/RIOT,smlng/RIOT,jasonatran/RIOT,kYc0o/RIOT,gebart/RIOT,OlegHahm/RIOT,toonst/RIOT,gebart/RIOT,RIOT-OS/RIOT,cladmi/RIOT,OTAkeys/RIOT,gebart/RIOT,ant9000/RIOT,lazytech-org/RIOT,smlng/RIOT,A-Paul/RIOT,OTAkeys/RIOT,aeneby/RIOT,basilfx/RIOT,rfuentess/RIOT,mtausig/RIOT,kYc0o/RIOT,authmillenon/RIOT,basilfx/RIOT,aeneby/RIOT,toonst/RIOT,mtausig/RIOT,mfrey/RIOT,aeneby/RIOT,rfuentess/RIOT,lazytech-org/RIOT,x3ro/RIOT,RIOT-OS/RIOT,basilfx/RIOT,BytesGalore/RIOT,smlng/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,toonst/RIOT,lazytech-org/RIOT,miri64/RIOT,OlegHahm/RIOT,miri64/RIOT,mtausig/RIOT,josephnoir/RIOT,authmillenon/RIOT,yogo1212/RIOT,mtausig/RIOT,BytesGalore/RIOT,josephnoir/RIOT,OlegHahm/RIOT,ant9000/RIOT,BytesGalore/RIOT,cladmi/RIOT,jasonatran/RIOT,mfrey/RIOT,biboc/RIOT,kaspar030/RIOT,yogo1212/RIOT,BytesGalore/RIOT,basilfx/RIOT,OTAkeys/RIOT,basilfx/RIOT,lazytech-org/RIOT,x3ro/RIOT,miri64/RIOT,OlegHahm/RIOT,x3ro/RIOT,toonst/RIOT,kaspar030/RIOT,kaspar030/RIOT,jasonatran/RIOT