sparc32: fix sparse warning in ptrace_32.c

Fix following warning:
ptrace_32.c:444:16: warning: symbol 'syscall_trace' was not declared. Should it be static?

Add declaration in kernel.h

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>