powerpc: Turn on BPF_JIT in ppc64_defconfig

Matt added BPF_JIT support in commit 0ca87f05, but currently none of our
defconfigs build it. Turn that sucker on.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>