install-lib-links: don't depend on .libs directory

install-lib-links: don't depend on .libs directory

This snippet can be included in Makefiles that may, depending on the
project configuration, not actually build any installable libraries.

In that case we don't have anything to depend on and this part of
the makefile may be executed before the .libs directory is created,
so do not depend on it being there.

Cc: "10.3 10.4 10.5" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>