tools/misc: Fix linkage of libxenstore

tools/misc: Fix linkage of libxenstore

* xen-mfndump doesn't use xenstore at all.  Don't link against it.

* xen-hptool can include the correct header rather than externing itself a
  single function.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>