md4: fix license
As ccanlint now says:
Source files don't contain incompatible licenses (license_file_compat): FAIL
/home/rusty/devel/cvs/ccan/ccan/md4/md4.c:Found boilerplate for license 'GPLv2+' which is incompatible with 'LGPLv2+'
This is actually GPL code!
Add LICENSE link, too.