devel/libunicode: properly register iconv dependency

Avoid accidentally using converters/libiconv symbols when libc is
desired by passing -DLIBICONV_PLUG during build. This fixes the
following error in consumers:

/usr/local/lib/libunicode.so.0: undefined reference to `libiconv'
/usr/local/lib/libunicode.so.0: undefined reference to `libiconv_close'
/usr/local/lib/libunicode.so.0: undefined reference to `libiconv_open'

PR: 206966
Submitted by: vvd@unislabs.com
Approved by: portmgr blanket
MFH: 2016Q1