xous: include prelude to define c_int

With the latest changes, libstd no longer builds:

```
   Compiling libc v0.2.170 (/opt/Xous/libc)
error[E0412]: cannot find type `c_int` in this scope
  --> /opt/Xous/libc/src/xous.rs:17:20
   |
17 | pub const INT_MIN: c_int = -2147483648;
   |                    ^^^^^ not found in this scope
   |
help: consider importing one of these type aliases
   |
5  + use crate::c_int;
   |
5  + use rustc_std_workspace_core::ffi::c_int;
   |

error[E0412]: cannot find type `c_int` in this scope
  --> /opt/Xous/libc/src/xous.rs:18:20
   |
18 | pub const INT_MAX: c_int = 2147483647;
   |                    ^^^^^ not found in this scope
   |
help: consider importing one of these type aliases
   |
5  + use crate::c_int;
   |
5  + use rustc_std_workspace_core::ffi::c_int;
   |

For more information about this error, try `rustc --explain E0412`.
error: could not compile `libc` (lib) due to 2 previous errors
```

Include the prelude to define `c_int` and fix the build on rust `main`.

Signed-off-by: Sean Cross <[email protected]>

(backport <#4304>)
(cherry picked from commit fd66a07)

Author: xobs

src/xous.rs

@@ -1,5 +1,7 @@
 //! Xous C type definitions
 
+use crate::prelude::*;
+
 pub type intmax_t = i64;
 pub type uintmax_t = u64;