Change license statement on sha1.c to GPLv2.

This might seem presumptuous but is actually legitimate. The version
of sha1.c here is derived from cipher/sha1.c from the GnuPG 1.4 stable
branch in 2012. Since October 2007, all GnuPG releases have been made
under the GPLv3, but the earlier releases were under GPLv2 and there
do not appear to have been any changes to sha1.c between 2007 and
2012. So the same file was previously released under the GPLv2.

The relevant commit from the GnuPG Git repository is 9a2a818887b4d36c;
this commit is essentially a revert of the part of that commit that
applies to cipher/sha1.c.

This fixes #479.

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