use license file from gnu.org

The current license file is missing the preamble and the instructions
for how to use it on your own programs, which are an integral part
of the GPL and are not supposed to be omitted.

Explanation here:
https://www.gnu.org/licenses/old-licenses/gpl-2.0-faq.html#GPLOmitPreamble

File downloaded from:
https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt