distro: Use (guix licenses) instead of strings.

* distro/packages/acl.scm, distro/packages/attr.scm,
  distro/packages/autotools.scm, distro/packages/base.scm,
  distro/packages/bash.scm, distro/packages/bdw-gc.scm,
  distro/packages/bison.scm, distro/packages/bootstrap.scm,
  distro/packages/compression.scm, distro/packages/cpio.scm,
  distro/packages/ddrescue.scm, distro/packages/ed.scm,
  distro/packages/flex.scm, distro/packages/gawk.scm,
  distro/packages/gdbm.scm, distro/packages/gettext.scm,
  distro/packages/gnupg.scm, distro/packages/gnutls.scm,
  distro/packages/gperf.scm, distro/packages/guile.scm,
  distro/packages/help2man.scm, distro/packages/less.scm,
  distro/packages/libffi.scm, distro/packages/libsigsegv.scm,
  distro/packages/libunistring.scm, distro/packages/linux.scm,
  distro/packages/lout.scm, distro/packages/lsh.scm,
  distro/packages/m4.scm, distro/packages/multiprecision.scm,
  distro/packages/nano.scm, distro/packages/ncurses.scm,
  distro/packages/nettle.scm, distro/packages/perl.scm,
  distro/packages/pkg-config.scm, distro/packages/pth.scm,
  distro/packages/readline.scm, distro/packages/recutils.scm,
  distro/packages/shishi.scm, distro/packages/system.scm,
  distro/packages/texinfo.scm, distro/packages/time.scm,
  distro/packages/wget.scm, distro/packages/which.scm,
  distro/packages/zile.scm: Use (guix licenses).