[mlir][bazel] Fix build after changes from #183856. (#184134)
This PR fixes the bazel build breakages introduced by #183856, which
introduced a new CMake flag that wasn't set in the bazel build, thus
leading to a placeholder not being replaced and a consequent syntax
error. The fix consists of setting this value to the default (`0`),
which is how similar flags are handled.
Signed-off-by: Ingo Müller <ingomueller@google.com>