kbuild: Delete .builtin-dtbs.S when running make clean

kbuild: Delete .builtin-dtbs.S when running make clean

The makefile tries to delete a file named ".builtin-dtb.S" but the file
created by scripts/Makefile.vmlinux is actually called ".builtin-dtbs.S".

Fixes: 654102df2ac2a ("kbuild: add generic support for built-in boot DTBs")
Cc: stable@vger.kernel.org
Signed-off-by: Charles Mirabile <cmirabil@redhat.com>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://patch.msgid.link/20260308044338.181403-1-cmirabil@redhat.com
[nathan: Small commit message adjustments]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>