cmake: fix compilation of clar-based unit tests

cmake: fix compilation of clar-based unit tests

The compilation of clar-based unit tests is broken because we do not
add the binary directory into which we generate the "clar-decls.h" and
"clar.suite" files as include directories. Instead, we accidentally set
up the source directory as include directory.

Fix this by including the binary directory instead of the source
directory. Furthermore, set up the include directories as PUBLIC instead
of PRIVATE such that they propagate from "unit-tests.lib" to the
"unit-tests" executable, which needs to include the same directory.

Reported-by: Ed Reel <edreel@gmail.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Taylor Blau <me@ttaylorr.com>