gnupg: depend on libiconv if appropriate

If iconv is built before gnupg, this package will link against it; if it is
built after this package, it will not be linked against it.  To make the
build reproducible, make sure that iconv is always built before this
package.

Signed-off-by: Bernd Kuhls <berndkuhls@hotmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>