LSM: SafeSetID: remove unused include

The include for asm/syscall.h was needed in a prior version of lsm.c
that checked return values of syscall_get_nr, but since we did away with
that part of the code this include is no longer necessary. Take out this
include since it breaks builds for certain architectures. We no longer
have any arch-specific code in SafeSetID.

Signed-off-by: Micah Morton <mortonm@chromium.org>
Signed-off-by: James Morris <james.morris@microsoft.com>