sparc: fix fallout from system.h split

Fixes this build error:

kernel/signal.c: In function 'ptrace_stop':
kernel/signal.c:1860:3: error: implicit declaration of function 'synchronize_user_stack' [-Werror=implicit-function-declaration]

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>