tools: libxl: link libxlu against libxl.

tools: libxl: link libxlu against libxl.

It uses libxl_defbool_set and must therefore be linked against the
right library.

Spotted by dpkg-shlibdeps and pointed out by Bastian Blank:

dpkg-shlibdeps: warning: symbol libxl_defbool_set used by debian/libxen-4.4/usr/lib/libxlutil-4.4.so found in none of the libraries

This required switching the make rule from $^ to an explicit
LIBXLU_OBJS since the former now includes libxenlight.so.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Cc: Bastian Blank <waldi@debian.org>