buildroot

libidn: fix build with gcc 7.x

Fixes:

  http://autobuild.buildroot.net/results/f7fc775584ad35349f0d66ed8632ab18f3859f9c/

Note: this problem wasn't seen until now because we were only testing
gcc 7.x/glibc, and the problematic code is not built when glibc is
used.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>