y2038: ipc: fix x32 ABI breakage

The correct type on x32 is 64-bit wide, same as for the other struct
members around it, so use  __kernel_long_t in place of the original
__kernel_time_t here, corresponding to the rest of the structure.

Fixes: caf5e32d ("y2038: ipc: remove __kernel_time_t reference from headers")
Reported-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>