buildroot

libgtk3: create icon-theme.cache for icon themes

It makes icon lookups faster, though it's not strictly necessary.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
[Thomas: enclsoe the hook definition in the condition.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>