ptr_ring: add include of linux/mm.h

Commit 0bf7800f1799 ("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: 0bf7800f1799 ("ptr_ring: try vmalloc() when kmalloc() fails")
Signed-off-by: Jesper Dangaard Brouer <[e-mail]>
Acked-by: Michael S. Tsirkin <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

include/linux/ptr_ring.h

···
 #include <linux/types.h>
 #include <linux/compiler.h>
 #include <linux/slab.h>
+#include <linux/mm.h>
 #include <asm/errno.h>
 #endif