Ensure we don't break conditional compilation

At the moment, any mistake passing the kernel configuration
for conditional compilation means we break things silently,
like initcall handling which implies panics or memory
corruption [1].

Let's prevent that from happening again.

[1] #23

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/src/lib.rs

@@ -5,6 +5,11 @@
 #![no_std]
 #![feature(allocator_api, alloc_error_handler)]
 
+// Ensure conditional compilation based on the kernel configuration works;
+// otherwise we may silently break things like initcall handling.
+#[cfg(not(CONFIG_HAS_RUST))]
+compile_error!("Missing kernel configuration for conditional compilation");
+
 extern crate alloc;
 
 use core::panic::PanicInfo;