tools/xenstore: fix linking libxenstore with ldl

tools/xenstore: fix linking libxenstore with ldl

Commit 448c03b3cbe1487 ("tools/xenstore: try to get minimum thread
stack size for watch thread") added a dependency to libdl to
libxenstore. Unfortunately the way it was added requires now all
users of libxenstore to specify "-ldl" when linking. This can be
avoided by linking libxenstore.so specifying "-ldl" as a trailing
option. So use APPEND_LDFLAGS instead of LDFLAGS for adding the
"-ldl" option when linking libxenstore.so.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Doug Goldstein <cardoe@cardoe.com>
Tested-by: Doug Goldstein <cardoe@cardoe.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>