Makefile.extrawarn: Move sign-compare from W=2 to W=3

This -Wsign-compare compiler warning can be very noisy
and most of the suggested conversions are unnecessary.

Make the warning W=3 so it's described under the
"can most likely be ignored" block.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/Makefile.extrawarn

@@ -66,7 +66,6 @@ KBUILD_CFLAGS += -Wnested-externs
 KBUILD_CFLAGS += -Wshadow
 KBUILD_CFLAGS += $(call cc-option, -Wlogical-op)
 KBUILD_CFLAGS += -Wmissing-field-initializers
-KBUILD_CFLAGS += -Wsign-compare
 KBUILD_CFLAGS += -Wtype-limits
 KBUILD_CFLAGS += $(call cc-option, -Wmaybe-uninitialized)
 KBUILD_CFLAGS += $(call cc-option, -Wunused-macros)
@@ -87,6 +86,7 @@ KBUILD_CFLAGS += -Wpacked
 KBUILD_CFLAGS += -Wpadded
 KBUILD_CFLAGS += -Wpointer-arith
 KBUILD_CFLAGS += -Wredundant-decls
+KBUILD_CFLAGS += -Wsign-compare
 KBUILD_CFLAGS += -Wswitch-default
 KBUILD_CFLAGS += $(call cc-option, -Wpacked-bitfield-compat)