This applies c11675 by Julian Seward from valgrind trunk
(svn://svn.valgrind.org/valgrind/trunk) to rustrt's included
copies of memcheck.h and valgrind.h, effectively backporting
the fix from the unreleased 3.6.2. The commit simply applies
the gcc 'unused' attribute to the relevant declarations.
This change allows compilation of the runtime code under
gcc 4.6 with -Werror, as the makefile currently requests.
