gtk-icon-cache: Fix GTKIC_CMD if-else condition

GTKIC_CMD variable gets the wrong assignation leading into a post
install script error. Fix if-else condition in GTKIC_CMD variable
to assign gtk4-update-icon-cache when GTKIC_VERSION is 4 but
gtk-update-icon-cache when is 3.

Also, rename gtk-update-icon-cache-3.0.0 to gtk-update-icon-cache-3.0
to match the gtk-update-icon-cache binary name deployed in
meta/recipes-gnome/gtk+/gtk+3.inc.

Signed-off-by: Daniel Gomez <daniel@qtec.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Robert Joslyn <robert.joslyn@redrectangle.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>