qt4: add eglibc-gconv-utf-16 to QtCore RRECOMMENDS when using glibc

This fixes the following warnings when running Qt applications:
QIconvCodec::convertFromUnicode: using Latin-1 for conversion, iconv_open failed
QIconvCodec::convertToUnicode: using Latin-1 for conversion, iconv_open failed

Qt's QString class stores strings internally using UTF-16 encoding.
The UTF-16 iconv module is needed to convert between the system's local
8-bit representation and QString's UTF-16 encoding.

For example, the following functions would be affected:
QString::fromLocal8Bit(...)
QString::toLocal8Bit(...)

If the UTF-16 iconv module couldn't be loaded, it would use Latin-1
encoding instead of the system's encoding for conversion.

[YOCTO #349]

Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>