include license and add missing dep

[noraj]

• including the license in the gem as MIT includes a copyright, it's unique to each gem
• lib/seccomp-tools.rb requires lib/seccomp-tools/asm/asm.rb
  • which requires lib/seccomp-tools/asm/compiler.rb
    • which requires lib/seccomp-tools/asm/sasm.tab.rb
      • which requires racc.

So even if racc may be unused at runtime because it's transitively required from the root level, the dependency either need must be provided in the gemspec or stop being required from root level.
For example, tasks/sasm.rake could require 'seccomp-tools/asm/sasm.tab', require 'seccomp-tools/asm/compiler' or require 'seccomp-tools/asm/asm' or whatever needed.

[david942j]

Thanks for the contribution!