kbuild: Add objtool to top-level clean target

kbuild: Add objtool to top-level clean target

Objtool is an integral part of the build, make sure it gets cleaned by
"make clean" and "make mrproper".

Fixes: 442f04c34a1a ("objtool: Add tool to perform compile-time stack metadata validation")
Reported-by: Jens Remus <jremus@linux.ibm.com>
Closes: https://lore.kernel.org/15f2af3b-be33-46fc-b972-6b8e7e0aa52e@linux.ibm.com
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Tested-by: Jens Remus <jremus@linux.ibm.com>
Link: https://patch.msgid.link/968faf2ed30fa8b3519f79f01a1ecfe7929553e5.1770759919.git.jpoimboe@kernel.org
[nathan: use Closes: instead of Link: per checkpatch.pl]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>