popt: add libiconv to popt.pc.in

Add ${LTLIBICONV} to popt.pc.in so applications such as shairport-sync
will know that they must link with -liconv when building statically

Fixes:
 - http://autobuild.buildroot.org/results/c5b0d1d2867e49c022a2ad971dd9f358ff0f3865



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>