kernel: import UserSliceRo and FileHandle in syscall/process.rs

The sys_mkns and sys_setns functions added in 48fc2f1c reference
UserSliceRo and FileHandle but the imports were not updated, causing
the build to fail with 'cannot find type' errors. This commit adds
the missing imports.

src/syscall/process.rs

@@ -27,7 +27,9 @@ use crate::{
     CurrentRmmArch,
 };
 
-use super::usercopy::UserSliceWo;
+use super::usercopy::{UserSliceRo, UserSliceWo};
+
+use crate::scheme::FileHandle;
 
 pub fn exit_this_context(excp: Option<syscall::Exception>, token: &mut CleanLockToken) -> ! {
     let mut close_files;