netdevsim: fix build if SKB_EXTENSIONS=n

netdevsim: fix build if SKB_EXTENSIONS=n

__skb_ext_put() is not declared if SKB_EXTENSIONS is not enabled, which
causes a build error:

drivers/net/netdevsim/netdev.c: In function 'nsim_forward_skb':
drivers/net/netdevsim/netdev.c:114:25: error: implicit declaration of function '__skb_ext_put'; did you mean 'skb_ext_put'? [-Werror=implicit-function-declaration]
  114 |                         __skb_ext_put(psp_ext);
      |                         ^~~~~~~~~~~~~
      |                         skb_ext_put
cc1: some warnings being treated as errors

Add a stub to fix the build.

Fixes: 7d9351435ebb ("netdevsim: drop PSP ext ref on forward failure")
Signed-off-by: Qingfang Deng <dqfext@gmail.com>
Link: https://patch.msgid.link/20260324140857.783-1-dqfext@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>