exiftool: add license
exiftool: add license

Sources:

- https://exiftool.org/#license

  > This is free software; you can redistribute it and/or modify it
  > under the same terms as Perl itself.

- https://dev.perl.org/licenses/

  > Perl5 (...) is free software;
  > you can redistribute it and/or modify it under the terms of either:
  > a) the GNU General Public License as published by the Free Software Foundation;
  >    either version 1, or (at your option) any later version, or
  > b) the "Artistic License".

nbari/homebrew-core,jvillard/homebrew-core,spaam/homebrew-core,cblecker/homebrew-core,dsXLII/homebrew-core,mfikes/homebrew-core,mahori/homebrew-core,jvillard/homebrew-core,uyjulian/homebrew-core,mbcoguno/homebrew-core,jabenninghoff/homebrew-core,stgraber/homebrew-core,chrisfinazzo/homebrew-core,dsXLII/homebrew-core,adam-moss/homebrew-core,Homebrew/homebrew-core,jvillard/homebrew-core,zyedidia/homebrew-core,adam-moss/homebrew-core,jabenninghoff/homebrew-core,cblecker/homebrew-core,Moisan/homebrew-core,sjackman/homebrew-core,mahori/homebrew-core,chrisfinazzo/homebrew-core,JCount/homebrew-core,JCount/homebrew-core,uyjulian/homebrew-core,Linuxbrew/homebrew-core,zyedidia/homebrew-core,makigumo/homebrew-core,nandub/homebrew-core,Moisan/homebrew-core,stgraber/homebrew-core,nandub/homebrew-core,mfikes/homebrew-core,mbcoguno/homebrew-core,spaam/homebrew-core,Linuxbrew/homebrew-core,lemzwerg/homebrew-core,Homebrew/homebrew-core,filcab/homebrew-core,mbcoguno/homebrew-core,nbari/homebrew-core,filcab/homebrew-core,sjackman/homebrew-core,lemzwerg/homebrew-core,makigumo/homebrew-core