libgtk3: enable gtk2-dependency for non host libgtk3

Building gtk-update-icon-cache is only needed for host-libgtk3. Disable
it for libgtk3.

Solve this error:
make[4]: Entering directory '/home/eksel/buildroot/output/build/libgtk3-3.12.2/gtk/native
Makefile:506: ../.deps/native_update_icon_cache-updateiconcache.Po: No such file or directory
make[4]: *** No rule to make target '../.deps/native_update_icon_cache-updateiconcache.Po'.
Stop.
make[4]: Leaving directory '/home/eksel/buildroot/output/build/libgtk3-3.12.2/gtk/native'
Makefile:6305: recipe for target 'gtkbuiltincache.h' failed

Signed-off-by: Hadrien Boutteville <hadrien.boutteville@gmail.com>