Fix the example provided in README to compile and run

[kaworu]

I've tested the patched example this way:

• building librdns

% mkdir build && cd build && cmake .. && make

• building the example from README

% vim test.c
% cc -g -c -I /usr/local/include -I ../include test.c
% cc -g test.o librdns_static.a -L /usr/local/lib -lev

• Finally launching the test:

% ./a.out ::1 53      
got result for host: google.com
got result for host: github.com
got result for host: www.ник.рф
got result for host: kernel.org
got result for host: freebsd.org
%

For info:

% uname -a
FreeBSD kaworu.ch 10.0-RELEASE-p9 FreeBSD 10.0-RELEASE-p9 #0: Mon Sep 15 14:35:52 UTC 2014     [email protected]:/usr/obj/usr/src/sys/GENERIC  amd64
% cc --version
FreeBSD clang version 3.3 (tags/RELEASE_33/final 183502) 20130610
Target: x86_64-unknown-freebsd10.0
Thread model: posix