__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>
---
 include/linux/skbuff.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 9cc98f850f1d..fbfa9852e82a 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -5106,6 +5106,7 @@ static inline bool skb_has_extensions(struct sk_buff *skb)
 	return unlikely(skb->active_extensions);
 }
 #else
+static inline void __skb_ext_put(struct skb_ext *ext) {}
 static inline void skb_ext_put(struct sk_buff *skb) {}
 static inline void skb_ext_reset(struct sk_buff *skb) {}
 static inline void skb_ext_del(struct sk_buff *skb, int unused) {}
-- 
2.43.0