alloc: ensure no_global_oom_handling builds are warning-free

Rust 1.62.0 introduced a couple new `unused_imports` warnings
in `no_global_oom_handling` builds, making a total of 5 warnings.

To avoid accumulating more over time, let's keep the builds
warning-free. This ensures projects compiling `alloc` without
infallible allocations do not see the warnings in the future
and that they can keep enabling `-Dwarnings`.

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

Author: ojeda

src/test/run-make-fulldeps/alloc-no-oom-handling/Makefile

@@ -1,4 +1,4 @@
 -include ../tools.mk
 
 all:
-	$(RUSTC) --edition=2021 --crate-type=rlib ../../../../library/alloc/src/lib.rs --cfg no_global_oom_handling
+	$(RUSTC) --edition=2021 -Dwarnings --crate-type=rlib ../../../../library/alloc/src/lib.rs --cfg no_global_oom_handling