kbuild: Do not delete empty files in make distclean

Commit 3d64b449 introduced an empty file under
arch/arm/mach-zynq/board_dt.c. While this was not intended and the file
was removed from the tree by a later commit, we really should only match
junk files by known name patterns and not their size.

Reported-by: David Howells <dhowells@redhat.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>