Re-add the test for primitive_types

This was added in d4da6c8 ("Move testing of primitive types from
std"), with cherry pick 69ce895. However, I accidentally reverted
this part of the patch in 65c9072 ("added wireless struct and
constants to Linux"), the cherry pick for 1ee94df.

Add this back here.

Author: tgross35

libc-test/Cargo.toml

@@ -88,3 +88,8 @@ harness = true
 name = "semver"
 path = "test/semver.rs"
 harness = false
+
+[[test]]
+name = "primitive_types"
+path = "test/primitive_types.rs"
+harness = true