tools/libs: Fix headers.chk logic

tools/libs: Fix headers.chk logic

c/s 4664034cd dropped the $(LIBHEADERSGLOB) dependency for the headers.chk
rule, without replacing it.

As headers.chk uses $^, a typical build looks like:

  andrewcoop@andrewcoop:/local/xen.git$ make -C tools/libs/devicemodel/
  make: Entering directory '/local/xen.git/tools/libs/devicemodel'
  for i in ; do \
      gcc -x c -ansi -Wall -Werror -I/local/xen.git/tools/libs/devicemodel/../../../tools/include \
            -S -o /dev/null $i || exit 1; \
      echo $i; \
  done >headers.chk.new
  mv headers.chk.new headers.chk

i.e. with an empty for loop.

Reinsert a $(LIBHEADERS) dependency, so more than just the $(AUTOINCS) get
checked.

Fixes: 4664034cd ("tools/libs: move official headers to common directory")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>