dmidecode: add license

pkgs/os-specific/linux/dmidecode/default.nix

···
   meta = with stdenv.lib; {
     homepage = http://www.nongnu.org/dmidecode/;
     description = "A tool that reads information about your system's hardware from the BIOS according to the SMBIOS/DMI standard";
+    license = licenses.gpl2Plus;
     platforms = platforms.linux;
   };
 }