[IA64] fix compile error in arch/ia64/mm/extable.c

ad6561df

 ("module: trim exception table on init
free.") put a bogus trim_init_extable() function into ia64 which didn't compile.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Tony Luck <tony.luck@intel.com>