Since commit d18692c libxml2 is linked against iconv. Now libiio needs
access to iconv as well. Without it the build fails.

[ 32%] Building C object CMakeFiles/iio.dir/lock.c.o
[ 34%] Building C object CMakeFiles/iio.dir/iiod-client.c.o
[ 37%] Linking C shared library libiio.so
/builder/shared-workdir/build/sdk/staging_dir/toolchain-aarch64_generic_gcc-8.4.0_musl/bin/../lib/gcc/aarch64-openwrt-linux-musl/8.4.0/../../../../aarch64-openwrt-linux-musl/bin/ld: cannot find -liconv
collect2: error: ld returned 1 exit status

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>