rust: fix double build of kernel.o

Put build_error.o in `extra-y` if it's not in `obj-y`
so it's always in `targets`.

Reported-by: Sven Van Asbroeck <thesven73@gmail.com>
Signed-off-by: Gary Guo <gary@garyguo.net>

Author: nbdd0121

rust/Makefile

@@ -9,7 +9,9 @@ extra-$(CONFIG_RUST) += bindings_generated.rs
 obj-$(CONFIG_RUST) += alloc.o kernel.o
 extra-$(CONFIG_RUST) += exports_alloc_generated.h exports_kernel_generated.h
 
-ifndef CONFIG_RUST_BUILD_ASSERT_DENY
+ifdef CONFIG_RUST_BUILD_ASSERT_DENY
+extra-$(CONFIG_RUST) += build_error.o
+else
 obj-$(CONFIG_RUST) += build_error.o
 endif