powerpc/fadump: Fix build break when CONFIG_PROC_VMCORE=n

The fadump code calls vmcore_cleanup() which only exists if
CONFIG_PROC_VMCORE=y. We don't want to depend on CONFIG_PROC_VMCORE,
because it's user selectable, so just wrap the call in an #ifdef.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>