glib: add missing dependencies for ptest

Some of the GLib tests that we run under ptest are missing runtime dependencies.

iochannel-test needs the EUC-JP encoding.

convert needs the ISO-8859-1 and ISO-8859-15 encodings.

contenttype needs shared-mime-info.

Add these to the RDEPENDS so that these tests pass.

[ YOCTO #5696 ]

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