malloc debug: fix compile error when enable macro MALLOC_DEBUG > 1

malloc debug: fix compile error when enable macro MALLOC_DEBUG > 1.

this is because commit e9c4fe93 has change the struct malloc_chunk's member "size" to "mchunk_size".

the reproduction is like that:
setp1: modify related Makefile.
vim ../glibc/malloc/Makefile
CPPFLAGS-malloc.o += -DMALLOC_DEBUG=2

step2: ../configure --prefix=/usr
       make -j32

this will cause the compile error:
/home/liqingqing/glibc_upstream/buildglibc/malloc/malloc.o
In file included from malloc.c:1899:0:
arena.c: In function 'dump_heap':
arena.c:422:58: error: 'struct malloc_chunk' has no member named 'size'
       fprintf (stderr, "chunk %p size %10lx", p, (long) p->size);
                                                          ^~
arena.c:428:17: error: 'struct malloc_chunk' has no member named 'size'
       else if (p->size == (0 | PREV_INUSE))

Reviewed-by: DJ Delorie <dj@redhat.com>