Fix c_char on AIX

Refs: rust-lang/rust#122985

Author: taiki-e

src/unix/aix/mod.rs

@@ -1,4 +1,4 @@
-pub type c_char = i8;
+pub type c_char = u8;
 pub type caddr_t = *mut ::c_char;
 pub type clockid_t = ::c_longlong;
 pub type blkcnt_t = ::c_long;