licenses: Add common TeX and LaTeX licenses.

* guix/licenses.scm (knuth, lppl, lppl1.0+, lppl1.2, lppl1.2+, lppl1.3,
lppl1.3+, lppl1.3a, lppl1.3a+, lppl1.3b, lppl1.3b+, lppl1.3c, lppl1.3c+): New
variables.