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>

scripts/Makefile.headersinst

@@ -14,7 +14,15 @@ __headers:
 include scripts/Kbuild.include
 
 srcdir        := $(srctree)/$(obj)
-subdirs       := $(patsubst $(srcdir)/%/.,%,$(wildcard $(srcdir)/*/.))
+
+# When make is run under a fakechroot environment, the function
+# $(wildcard $(srcdir)/*/.) doesn't only return directories, but also regular
+# files. So, we are using a combination of sort/dir/wildcard which works
+# with fakechroot.
+subdirs       := $(patsubst $(srcdir)/%/,%,\
+		 $(filter-out $(srcdir)/,\
+		 $(sort $(dir $(wildcard $(srcdir)/*/)))))
+
 # caller may set destination dir (when installing to asm/)
 _dst          := $(if $(dst),$(dst),$(obj))