binder: Fix up Kconfig dependancy due to removal of .c code

When the .c binder code was removed, building the kernel if the rust
binder code was enabled, will cause the binder to not be built at all as
the option changed from .c to .rs which is probably not what the
original build wanted.

Fix this up by renaming the option back to the _RUST version.  If in the
future, that suffix wants to be dropped, we can do so and then it will
be prompted for a choice again, but for this release cycle, it should
stay as-is.

Tested-by: Carlos Llamas <cmllamas@google.com>
Link: https://patch.msgid.link/2026091743-pony-prewar-735b@gregkh
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>