VS: Fix assertion failure on INCLUDE_DIRECTORIES in INTERFACE libraries

Since commit 43919131 (Add INTERFACE libraries to generated
buildsystem if they have SOURCES, 2020-07-20, v3.19.0-rc1~346^2~1), the
VS generator may process INTERFACE libraries.  Avoid code paths in the
generator that process include directories because they should not be
used by INTERFACE libraries since they do not compile anything.

Fixes: #22494