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>