pppd: Enable IPV6 by default and fix some warnings

pppd: Enable IPV6 by default and fix some warnings

Fixes these warnings:

ipv6cp.c: In function ‘ipv6_check_options’:
ipv6cp.c:1106:2: warning: implicit declaration of function ‘ether_to_eui64’ [-Wimplicit-function-declaration]
ipv6cp.c:1133:2: warning: implicit declaration of function ‘exit’ [-Wimplicit-function-declaration]
ipv6cp.c:1133:2: warning: incompatible implicit declaration of built-in function ‘exit’ [enabled by default]

Signed-off-by: Paul Mackerras <paulus@samba.org>