Fix Valgrind annotations so they can actually be built

Fix Valgrind annotations so they can actually be built

The AC_CHECK_HEADER() test for <valgrind/memcheck.h> will never result
in HAVE_VALGRIND_MEMCHECK_H being defined, so ibverbs.h will never
include <valgrind/memcheck.h> and Valgrind annotations will never actually
get built.  Fix this by adding an AC_DEFINE() of HAVE_VALGRIND_MEMCHECK_H
if the header is found.

Pointed out by Jeff Squyres <jsquyres@cisco.com>.

Signed-off-by: Roland Dreier <rolandd@cisco.com>