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

arch/sparc/include/asm/ptrace.h

@@ -165,6 +165,7 @@ struct sparc_stackf {
 #ifdef __KERNEL__
 
 #include <linux/threads.h>
+#include <asm/switch_to.h>
 
 static inline int pt_regs_trap_type(struct pt_regs *regs)
 {