package/tvheadend: Add optional dependency to libiconv

To get reproducable builds depend on libiconv if needed.

tvheadend configure either shows

  checking for cc libiconv ...                      fail

or

  checking for cc libiconv ...                      ok

depending on the presence of libiconv, and gets linked

output/host/usr/bin/i586-buildroot-linux-uclibc-readelf -a output/target/usr/bin/tvheadend | grep NEEDED | grep iconv
 0x00000001 (NEEDED)                     Shared library: [libiconv.so.2]

Please note that libiconv is not a hard-dependency.

Tested with minimal defconfig

BR2_TOOLCHAIN_BUILDROOT_LARGEFILE=y
BR2_TOOLCHAIN_BUILDROOT_INET_IPV6=y
BR2_PACKAGE_LIBICONV=y
BR2_PACKAGE_TVHEADEND=y

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>