fontcache.bbclass: add fontconfig-utils runtime dependency

In an image recipe, you can get a warning, for example:
WARNING: The postinstall intercept hook 'update_font_cache' failed (exit code: 1)! See log for details!
WARNING: The postinstalls for the following packages will be postponed for first boot: ttf-dejavu-sans-mono

(because /usr/bin/fc-cache is missing)

In OE-core, rdepend is correctly done in each recipe:
- ttf-fonts/liberation-fonts
- ttf-fonts/ttf-bitstream-vera

In meta-OE, rdepend is not done but packagegroup-fonts-truetype.bb includes fontconfig-utils:
- ttf-fonts/ttf-arphic-uming
- ttf-fonts/ttf-dejavu
- ttf-fonts/ttf-droid
- ttf-fonts/ttf-gentium
- ttf-fonts/ttf-hunkyfonts
- ttf-fonts/ttf-inconsolata
- ttf-fonts/ttf-liberation
- ttf-fonts/ttf-mplus
- ttf-fonts/ttf-sazanami
- ttf-fonts/ttf-ubuntu-font-family
- ttf-fonts/ttf-wqy-zenhei

Signed-off-by: Matthieu Crapet <Matthieu.Crapet@ingenico.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>