um: remove "extern" from implementation of sigchld_handler

There is no need to mark the function as extern in the implementation.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202506051226.X8r7X5aa-lkp@intel.com/


Fixes: 8420e08f ("um: Track userspace children dying in SECCOMP mode")
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20250605050325.1077208-2-benjamin@sipsolutions.net


Signed-off-by: Johannes Berg <johannes.berg@intel.com>