net: psp: select CONFIG_SKB_EXTENSIONS

net: psp: select CONFIG_SKB_EXTENSIONS

psp now uses skb extensions, failing to build when that is disabled:

In file included from include/net/psp.h:7,
                 from net/psp/psp_sock.c:9:
include/net/psp/functions.h: In function '__psp_skb_coalesce_diff':
include/net/psp/functions.h:60:13: error: implicit declaration of function 'skb_ext_find'; did you mean 'skb_ext_copy'? [-Wimplicit-function-declaration]
   60 |         a = skb_ext_find(one, SKB_EXT_PSP);
      |             ^~~~~~~~~~~~
      |             skb_ext_copy
include/net/psp/functions.h:60:31: error: 'SKB_EXT_PSP' undeclared (first use in this function)
   60 |         a = skb_ext_find(one, SKB_EXT_PSP);
      |                               ^~~~~~~~~~~
include/net/psp/functions.h:60:31: note: each undeclared identifier is reported only once for each function it appears in
include/net/psp/functions.h: In function '__psp_sk_rx_policy_check':
include/net/psp/functions.h:94:53: error: 'SKB_EXT_PSP' undeclared (first use in this function)
   94 |         struct psp_skb_ext *pse = skb_ext_find(skb, SKB_EXT_PSP);
      |                                                     ^~~~~~~~~~~
net/psp/psp_sock.c: In function 'psp_sock_recv_queue_check':
net/psp/psp_sock.c:164:41: error: 'SKB_EXT_PSP' undeclared (first use in this function)
  164 |                 pse = skb_ext_find(skb, SKB_EXT_PSP);
      |                                         ^~~~~~~~~~~

Select the Kconfig symbol as we do from its other users.

Fixes: 6b46ca260e22 ("net: psp: add socket security association code")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Daniel Zahka <daniel.zahka@gmail.com>
Link: https://patch.msgid.link/20260216105500.2382181-1-arnd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>