rust_binder: Fix build failure if !CONFIG_COMPAT

rust_binder: Fix build failure if !CONFIG_COMPAT

The bindgen utility cannot handle "#define compat_ptr_ioctl NULL" in the
C header, so we need to handle this case on our own.

Simply skip this field in the initializer when !CONFIG_COMPAT as the
SAFETY comment above this initializer implies this is allowed.

Reported-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Closes: https://lore.kernel.org/all/CANiq72mrVzqXnAV=Hy2XBOonLHA6YQgH-ckZoc_h0VBvTGK8rA@mail.gmail.com/
Signed-off-by: Xi Ruoyao <xry111@xry111.site>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20251209125029.1117897-1-xry111@xry111.site
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>