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

Following commit e88ca24 ("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>

Author: ojeda

Makefile

@@ -901,9 +901,6 @@ stackp-flags-$(CONFIG_STACKPROTECTOR_STRONG)      := -fstack-protector-strong
 
 KBUILD_CFLAGS += $(stackp-flags-y)
 
-KBUILD_RUSTFLAGS-$(CONFIG_WERROR) += -Dwarnings
-KBUILD_RUSTFLAGS += $(KBUILD_RUSTFLAGS-y)
-
 ifdef CONFIG_FRAME_POINTER
 KBUILD_CFLAGS	+= -fno-omit-frame-pointer -fno-optimize-sibling-calls
 KBUILD_RUSTFLAGS += -Cforce-frame-pointers=y

scripts/Makefile.extrawarn

@@ -217,5 +217,6 @@ endif
 ifneq ($(findstring e, $(KBUILD_EXTRA_WARN))$(CONFIG_WERROR),)
 
 KBUILD_CPPFLAGS		+= -Werror
+KBUILD_RUSTFLAGS	+= -Dwarnings
 
 endif