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>