core: ensure no_fp_fmt_parse builds are warning-free

Rust recently introduced a new `unused_imports` warning
in `no_fp_fmt_parse` builds, which was fixed in
#105434.

To avoid accumulating more over time, let's keep the builds
warning-free. This ensures projects compiling `core` with
this custom config do not see the warnings in the future
and that they can keep enabling `-Dwarnings`.

Similarly, #98652 did it
for `alloc`'s `no_global_oom_handling`.

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

Author: ojeda

src/test/run-make-fulldeps/core-no-fp-fmt-parse/Makefile

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