qt4-x11-free: depend on ICU

qt4-x11-free: depend on ICU

ICU presence is auto-detected at configure time and until recently (e68850 and
d61230) was pulled into most builds through harfbuzz and beecrypt.  Now it's
floating and this leads to build failures.

As in all likelihood the majority of people were building this with ICU enabled,
add an explicit dependency.

(From OE-Core rev: 46dcec6fd455584d9b5d0d7ff1e5b36fbe5a2d62)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>