tools/libs: Fix build following c/s 56dccee3f

tools/libs: Fix build following c/s 56dccee3f

Travis reports:

  make subdirs-install
  make[2]: Entering directory `/home/travis/build/andyhhp/xen/tools'
  make[3]: Entering directory `/home/travis/build/andyhhp/xen/tools'
  make -C libs install
  make[4]: Entering directory `/home/travis/build/andyhhp/xen/tools/libs'
  make[5]: Entering directory `/home/travis/build/andyhhp/xen/tools/libs'
  make -C toolcore install
  make[6]: Entering directory `/home/travis/build/andyhhp/xen/tools/libs/toolcore'
  make libs
  make[7]: Entering directory`/home/travis/build/andyhhp/xen/tools/libs/toolcore'
  for i in include/xentoolcore.h include/xentoolcore_internal.h; do \
          gcc -x c -ansi -Wall -Werror -I<snip>/xen/tools/libs/toolcore/../../../tools/include \
                    -S -o /dev/null $i || exit 1; \
                        echo $i; \
                        done >headers.chk.new
  include/xentoolcore_internal.h:30:31: fatal error: _xentoolcore_list.h: No such file or directory
   #include "_xentoolcore_list.h"
                                 ^
  compilation terminated.
  make[7]: *** [headers.chk] Error 1

The problem is that xentoolcore_internal.h includes _xentoolcore_list.h which
hasn't been generated yet.

The toolcore headers.chk rule (unlike the other libraries) had an additional
dependency against $(AUTOINCS), which forced the headers to be generated
first.  Replicate this in the common libs.mk

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>