eth: mlx4: select PAGE_POOL

eth: mlx4: select PAGE_POOL

With commit 8533b14b3d65 ("eth: mlx4: create a page pool for Rx") mlx4
started using functions guarded by PAGE_POOL. This change introduced
build errors when CONFIG_MLX4_EN is set but CONFIG_PAGE_POOL is not:

  ld: vmlinux.o: in function `mlx4_en_alloc_frags':
  en_rx.c:(.text+0xa5eaf9): undefined reference to `page_pool_alloc_pages'
  ld: vmlinux.o: in function `mlx4_en_create_rx_ring':
  (.text+0xa5ee91): undefined reference to `page_pool_create'

Make MLX4_EN select PAGE_POOL to fix the ml;x4 build errors.

Fixes: 8533b14b3d65 ("eth: mlx4: create a page pool for Rx")
Signed-off-by: Greg Thelen <gthelen@google.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20250401015315.2306092-1-gthelen@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>