ptr_ring: add include of linux/mm.h

Commit 0bf7800f ("ptr_ring: try vmalloc() when kmalloc() fails")
started to use kvmalloc_array and kvfree, which are defined in mm.h,
the previous functions kcalloc and kfree, which are defined in slab.h.

Add the missing include of linux/mm.h.  This went unnoticed as other
include files happened to include mm.h.

Fixes: 0bf7800f ("ptr_ring: try vmalloc() when kmalloc() fails")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>