Do not allow SHORT_DBG_HDRS if KEEP_BACK_PTRS or MAKE_BACK_GRAPH

Do not allow SHORT_DBG_HDRS if KEEP_BACK_PTRS or MAKE_BACK_GRAPH

Otherwise GC_HAS_DEBUG_INFO (defined as (p&1)) might return true
if a non-pointer is stored at the beginning of the tested object
leading further to its corruption by GC_store_back_pointer.
See issue #125 for details.

* include/private/dbg_mlc.h [KEEP_BACK_PTRS || MAKE_BACK_GRAPH]
(GC_HAS_DEBUG_INFO): Add #error (with the appropriate message) in
case of SHORT_DBG_HDRS defined.