postgresql: fix IPv6 detection for uclibc compile

Assist configure IPv6 support detection for uclibc,
fixes the following uclibc compile failure:

auth.c: In function ‘CheckRADIUSAuth’:
auth.c:2379:24: error: ‘in6addr_any’ undeclared (first use in this function)
  localaddr.sin6_addr = in6addr_any;

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>