Since commit d5470d14 ("kbuild: re-implement Makefile.headersinst
without recursion"), headers_install emits an ugly warning.

$ make headers_install
  [ snip ]
  UPD     include/generated/uapi/linux/version.h
find: ‘./include/uapi/Kbuild’: No such file or directory
  HDRINST usr/include/video/uvesafb.h
    ...

This happens for GNU Make <= 4.2.1

When I wrote that commit, I missed this warning because I was using the
state-of-the-art Make version compiled from the git tree.

$(wildcard $(src)/*/) is intended to match to only existing directories
since it has a trailing slash, but actually matches to regular files too.
(include/uapi/Kbuild in this case)

This is a bug of GNU Make, and was fixed by:

| commit b7acb10e86dc8f5fdf2a2bbd87e1059c315e31d6
| Author: spagoveanu@gmail.com <spagoveanu@gmail.com>
| Date:   Wed Jun 20 02:03:48 2018 +0300
|
|    * src/dir.c: Preserve glob d_type field

We need to cater to old Make versions. Add '$(filter %/,...) to filter
out the regular files.

Fixes: d5470d14

 ("kbuild: re-implement Makefile.headersinst without recursion")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>