Kbuild: fix file name in comment about extra gcc checks

Extra gcc checks like W=1 were moved to scripts/Makefile.exrawarn,
so the file name in comment needs to be fixed.
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -707,7 +707,7 @@ KBUILD_CFLAGS += $(call cc-option, -fcatch-undefined-behavior)
 else
 
 # These warnings generated too much noise in a regular build.
-# Use make W=1 to enable them (see scripts/Makefile.build)
+# Use make W=1 to enable them (see scripts/Makefile.extrawarn)
 KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
 KBUILD_CFLAGS += $(call cc-disable-warning, unused-const-variable)
 endif