ppp: Add RSA-MD in LICENSE

ppp package has "RSA Data Security" license text in
Message-Digest Algorithm source file ppp-md5.c and ppp-md4.c
Add RSA-MD in LICENSE field for ppp package

Signed-off-by: Poonam Jadhav <poonam.jadhav@kpit.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 9c58193c)
Signed-off-by: Steve Sakoman <steve@sakoman.com>