htp.pc.in: move libiconv in Libs.private
htp.pc.in: move libiconv in Libs.private

GNU libiconv is a private library so move it from Libs to Libs.private
in htp.pc.in

Signed-off-by: Fabrice Fontaine <7d88633730b8b85c1516e612893df72eae9cd341@gmail.com>

OISF/libhtp,OISF/libhtp,OISF/libhtp,OISF/libhtp,OISF/libhtp,OISF/libhtp,OISF/libhtp