kmc-solid: Add a stub implementation of is_terminal

Copied from `unsupported/io.rs`. Fixes build failure.

library/std/src/sys/solid/io.rs

@@ -75,3 +75,7 @@ impl<'a> IoSliceMut<'a> {
         unsafe { slice::from_raw_parts_mut(self.vec.iov_base as *mut u8, self.vec.iov_len) }
     }
 }
+
+pub fn is_terminal<T>(_: &T) -> bool {
+    false
+}