Fix c_char on AIX

Refs: rust-lang#122985

Author: taiki-e

core/src/ffi/mod.rs

@@ -133,7 +133,8 @@ mod c_char_definition {
                 any(target_arch = "aarch64", target_arch = "riscv64")
             ),
             all(target_os = "nto", target_arch = "aarch64"),
-            target_os = "horizon"
+            target_os = "horizon",
+            target_os = "aix",
         ))] {
             pub type c_char = u8;
         } else {