mlx4_core: Fix build break (missing include)

Commit 313abe55 ("mlx4_core: For 64-bit systems, vmap() kernel queue
buffers") caused this to pop up on powerpc allyesconfig, looks like a
missing include file:

    drivers/net/mlx4/alloc.c: In function 'mlx4_buf_alloc':
    drivers/net/mlx4/alloc.c:162: error: implicit declaration of function 'vmap'
    drivers/net/mlx4/alloc.c:162: error: 'VM_MAP' undeclared (first use in this function)
    drivers/net/mlx4/alloc.c:162: error: (Each undeclared identifier is reported only once
    drivers/net/mlx4/alloc.c:162: error: for each function it appears in.)
    drivers/net/mlx4/alloc.c:162: warning: assignment makes pointer from integer without a cast
    drivers/net/mlx4/alloc.c: In function 'mlx4_buf_free':
    drivers/net/mlx4/alloc.c:187: error: implicit declaration of function 'vunmap'

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Roland Dreier <rolandd@cisco.com>