CI: rfl: build the generated doctests

We were already generating the doctests, which should already catch most
issues with our hack around `--test-builder` and `--no-run`.

However, we were not building the result of that transformation, thus
build it for completeness and to ensure the hack may not have produced
something completely broken.

In the worst case, we can revert it.

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

Author: ojeda

src/ci/docker/scripts/rfl-build.sh

@@ -65,4 +65,5 @@ make -C linux LLVM=1 -j$(($(nproc) + 1)) \
 make -C linux LLVM=1 -j$(($(nproc) + 1)) \
     samples/rust/rust_minimal.o \
     samples/rust/rust_print.o \
-    drivers/net/phy/ax88796b_rust.o
+    drivers/net/phy/ax88796b_rust.o \
+    rust/doctests_kernel_generated.o