atm: hide unused procfs functions

[ Upstream commit fb1b7be9

 ]

When CONFIG_PROC_FS is disabled, the function declarations for some
procfs functions are hidden, but the definitions are still build,
as shown by this compiler warning:

net/atm/resources.c:403:7: error: no previous prototype for 'atm_dev_seq_start' [-Werror=missing-prototypes]
net/atm/resources.c:409:6: error: no previous prototype for 'atm_dev_seq_stop' [-Werror=missing-prototypes]
net/atm/resources.c:414:7: error: no previous prototype for 'atm_dev_seq_next' [-Werror=missing-prototypes]

Add another #ifdef to leave these out of the build.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230516194625.549249-2-arnd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>