pppd.mk installs radattr.so twice and omits radrealms.so

In pppd.mk the radattr.so plugin (for radiua) is installed twice (the second
install overwriting the first) but the radrealms.so plugin is omitted from
the install. This appears to be a copy-paste error.

Below is patch that corrects this.

signed-off by: Darcy Watkins <dwatkins@tranzeo.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>