blktap2/vhd: add -liconv when linking if using libiconv

blktap2/vhd: add -liconv when linking if using libiconv

If libiconv is detected on the system add -liconv when linking the
libvhd library.

If -liconv is not added when compiling libvhd with libiconv the
following error occours when linking vhd-util and vhd-update:

gcc     -o vhd-util vhd-util.o -Llib -lvhd
lib/libvhd.so: undefined reference to `libiconv_open'
lib/libvhd.so: undefined reference to `libiconv_close'
lib/libvhd.so: undefined reference to `libiconv'

Signed-off-by: Roger Pau Monne <roger.pau@entel.upc.edu>
Committed-by: Ian Jackson <ian.jackson.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>