perf MANIFEST: Add license files

The standalone tarballs should include the license files - both the
COPYING declaration as well as the text of GPLv2.

Signed-off-by: Michel Lind <michel@michel-slm.name>
Link: https://lore.kernel.org/r/Z0Zcx0WRqtlUYpgw@hyperscale.parallels


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>