pkg/heatshrink: define PKG_LICENSE to ISC-License
pkg/heatshrink: define PKG_LICENSE to ISC-License

This "ISC License" is the value found by github

https://github.com/atomicobject/heatshrink/blob/7d419e1fa4830d0b919b9b6a91fe2fb786cf3280/LICENSE

    A permissive license lets people do anything with your code with proper
    attribution and without warranty. The ISC license is functionally
    equivalent to the BSD 2-Clause and MIT licenses, removing some language
    that is no longer necessary.

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