kbuild: check the presence of include/generated/rustc_cfg

Since commit 2f7ab126 ("Kbuild: add Rust support"), Kconfig
generates include/generated/rustc_cfg, but its presence is not checked
in the top-level Makefile. It should be checked similarly to the C
header counterpart, include/generated/autoconf.h.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Tested-by: Miguel Ojeda <ojeda@kernel.org>