ding-libs: fix build with !locale

It needs libiconv if the toolchain doesn't have iconv support.

Fixes:
http://autobuild.buildroot.net/results/60f/60f85a2144b167f33ed674ff5979c1ef20bc0555/
http://autobuild.buildroot.net/results/76d/76d249f5a85da72cc53a3e66202c372dc33c9c38/
http://autobuild.buildroot.net/results/9f7/9f78043f549e09d020d69f45cd26c3e43a91d9dc/
http://autobuild.buildroot.net/results/8a5/8a586b781b6923444857ef58de659e5f0e8652ee/



And many more.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>