dlmalloc.c: Fix gcc alias warning

Fix these warnings:
dlmalloc.c: In function 'free':
dlmalloc.c:2507: warning: dereferencing pointer '({anonymous})' does break strict-aliasing rules
dlmalloc.c:2507: warning: dereferencing pointer '({anonymous})' does break strict-aliasing rules
dlmalloc.c:2507: warning: dereferencing pointer '({anonymous})' does break strict-aliasing rules

Some page(http://blog.worldofcoding.com/2010/02/solving-gcc-44-strict-aliasing-problems.html)
suggests adding __attribute__((__may_alias__)). Doing so makes the warnings go away.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Acked-by: Mike Frysinger <vapier@gentoo.org>

common/dlmalloc.c

@@ -1152,7 +1152,7 @@ struct malloc_chunk
   INTERNAL_SIZE_T size;      /* Size in bytes, including overhead. */
   struct malloc_chunk* fd;   /* double links -- used only if free. */
   struct malloc_chunk* bk;
-};
+} __attribute__((__may_alias__)) ;
 
 typedef struct malloc_chunk* mchunkptr;