kbuild: fix header installation under fakechroot environment

Since commit fcc8487d ("uapi: export all headers under uapi
directories") fakechroot make bindeb-pkg fails, mismatching files for
directories:
touch: cannot touch 'usr/include/video/uvesafb.h/.install': Not a
directory

This due to a bug in fakechroot:
when using the function $(wildcard $(srcdir)/*/.) in a makefile, under a
fakechroot environment, not only directories but also files are
returned.

To circumvent that, we are using the functions:
$(sort $(dir $(wildcard $(srcdir)/*/))))

Fixes: fcc8487d ("uapi: export all headers under uapi directories")
Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>