_GNU_SOURCE needed for EAI_INPROGRESS

Define of _GNU_SOURCE is needed to be able to use EAI_INPROGRESS in
loop.c.

This patch fixes a build error

loop.c:334:17: error: ‘EAI_INPROGRESS’ undeclared (first use in this function)
        if(rc == EAI_INPROGRESS){

occuring with a glibc-2.27-based buildroot toolchain for sparc64

Target: sparc64-buildroot-linux-gnu
[...]
gcc version 6.4.0 (Buildroot 2018.05)

Source:
http://autobuild.buildroot.org/toolchains/tarballs/br-sparc64-full-2018.05.tar.bz2


Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>