Add non-functional workspace.excludes.

Once rust-lang/cargo#6745 is fixed, this
`exclude` section will work and do what we need, but for now it is a
no-op.

Cargo.toml

@@ -6,3 +6,7 @@ members = [
     "cubeb-sys",
     "systest"
 ]
+exclude = [
+  "cubeb-sys/libcubeb/src/cubeb-coreaudio-rs",
+  "cubeb-sys/libcubeb/src/cubeb-pulse-rs",
+]