gnome-themes: fix icon-name-mapping not found

Since libexecdir changed, we need also tweak for gnome-themes.
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commit f35b2e29d9bfa817e64b83df11ebbbfe0f4e8e5c
Author: Ross Burton <ross.burton@intel.com>
Date:   Tue Apr 30 20:35:54 2013 +0100

    bitbake: set default libexecdir to $prefix/libexec
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Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>