rust: build_error: export rust_build_error

Export the `rust_build_error` symbol so that kernel modules can
take advantage of the `build_error!` and `build_assert!` macros.

Otherwise, kernel modules would fail to compile if there is a call
under `RUST_BUILD_ASSERT_{ALLOW,WARN}`.

Fixes: b8b9cab ("Implement build-time error and assertion")
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/exports.c

@@ -19,3 +19,8 @@
 #include "exports_alloc_generated.h"
 #include "exports_bindings_generated.h"
 #include "exports_kernel_generated.h"
+
+// For modules using `rust/build_error.rs`.
+#ifndef CONFIG_RUST_BUILD_ASSERT_DENY
+EXPORT_SYMBOL_RUST_GPL(rust_build_error);
+#endif