kbuild: modpost: include .*.cmd files only when targets exist

If a build rule fails, the .DELETE_ON_ERROR special target removes the
target, but does nothing for the .*.cmd file, which might be corrupted.
So, .*.cmd files should be included only when the corresponding targets
exist.

Commit 392885ee ("kbuild: let fixdep directly write to .*.cmd
files") missed to fix up this file.

Fixes: 392885ee ("kbuild: let fixdep directly write to .*.cmd")
Cc: <stable@vger.kernel.org> # v5.0+
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>