Fix EXCLUDE_FROM_ALL on directory with an interface library

Since commit dc688857 (Pass EXCLUDE_FROM_ALL from directory to
targets, 2019-01-15, v3.14.0-rc1~83^2) we automatically forward the
`EXCLUDE_FROM_ALL` to targets as they are created.  This regressed
support for interface libraries on which the property is not allowed.

Skip forwarding the `EXCLUDE_FROM_ALL` property for interface libraries.
It is not needed on them because they do not participate in the
generated build system anyway.

Fixes: #18896
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