Remove the c_char_def workaround for OpenBSD

The exception was added after the PR with 0a02b94 ("Define c_char at
top-level...") was posted. Remove this skip since the same commit makes
it no longer relevant.

(backport <rust-lang#4202>)
(cherry picked from commit 2bb023e)

Author: tgross35

libc-test/build.rs

@@ -644,11 +644,6 @@ fn test_openbsd(target: &str) {
         }
     });
 
-    cfg.skip_type(move |ty| {
-        // `c_char_def` is always public but not always reexported.
-        ty == "c_char_def"
-    });
-
     cfg.type_name(move |ty, is_struct, is_union| {
         match ty {
             // Just pass all these through, no need for a "struct" prefix