multimedia/gavl: Fix build if port iconv installed

In some cases, if converters/libiconv is installed the port will fail
to link. This does not affect poudriere builds, but since iconv methods
are used, USES=iconv should be added to the Makefile anyways.

ld: error: undefined symbol: libiconv_open

referenced by charset.c
.libs/charset.o:(gavl_charset_converter_create)

Reported by: vvd via #freebsd-desktop