proxyarp: Use C library header files and CONFIG_IPV6

proxyarp: Use C library header files and CONFIG_IPV6

This replaces the use of Linux kernel header files (linux/ip.h,
linux/udp.h, linux/ipv6.h, and linux/icmpv6.h) with equivalent header
files from C library. In addition, ndisc_snoop.c is now built
conditionally on CONFIG_IPV6=y so that it is easier to handle hostapd
builds with toolchains that do not support IPv6 even if Hotspot 2.0 is
enabled in the build.

Signed-off-by: Jouni Malinen <j@w1.fi>