kbuild: rust: move `-Dwarnings` handling to `Makefile.extrawarn`

Following commit e88ca243 ("kbuild: consolidate warning flags
in scripts/Makefile.extrawarn"), move `-Dwarnings` handling into
`Makefile.extrawarn` like C's `-Werror`.

No functional change intended.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://lore.kernel.org/r/20250814-kbuild-werror-v2-3-c01e596309d2@linutronix.de


Signed-off-by: Nathan Chancellor <nathan@kernel.org>