valgrind: explicitly disable MPI2 support

We don't have support for this in OE-Core, so ensure we don't pick this
up from the build host if e.g. openmpi development files happen to be
installed there.

Fixes [YOCTO #3726].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>