GNU Make 3.81 fails in CONFIG_RUST=y builds.

  rust/Makefile:105: *** multiple target patterns.  Stop.
  make[1]: *** [prepare] Error 2
  make: *** [__sub-make] Error 2

The error message is unclear, but the reason is because the 'private'
keyword is only supported since GNU Make 3.82.

GNU Make 3.81 is still able to build the kernel when CONFIG_RUST is
disabled, but it might be a good timing to raise the minimal GNU Make
version. Perhaps, I am the last person who was testing GNU Make 3.81.

GNU Make 3.81 was released in 2006, GNU Make 3.82 in 2010.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>