tile: move declaration of sys_call_table to <asm/syscall.h>

When activating syscall tracing, kernel/trace/trace_syscalls.c doesn't
find sys_call_table because it includes <asm/syscall.h>, not
<asm/syscalls.h>. Also, looking at the other architectures, that is
probably where it should be.

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>