kbuild: Shuffle blank line to improve comment meaning

-Wunused-but-set-variable and -Wunused-const-variable are both disabled
for the same reason but there is a blank line between them and no blank
line between -Wno-unused-const-variable and the block.

Shuffle the new line so that it is clear that the comment applied to
both flags and the next block is separate from them.

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