Don't mark `DEBUG_EVENT` struct as `repr(packed)`

That would give it alignment of 1 which is ABI-incompatible with its C
definition.

library/std/src/process/tests.rs

@@ -386,7 +386,7 @@ fn test_interior_nul_in_env_value_is_error() {
 fn test_creation_flags() {
     use crate::os::windows::process::CommandExt;
     use crate::sys::c::{BOOL, DWORD, INFINITE};
-    #[repr(C, packed)]
+    #[repr(C)]
     struct DEBUG_EVENT {
         pub event_code: DWORD,
         pub process_id: DWORD,