arm64: Include linux/ptrace.h in asm/compat.h

Commit 9b064fc3f9 (new helper:
compat_user_stack_pointer()) introduces a call to current_pt_regs()
which is defined in linux/ptrace.h, not currently included asm/compat.h.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/compat.h

@@ -23,6 +23,7 @@
  */
 #include <linux/types.h>
 #include <linux/sched.h>
+#include <linux/ptrace.h>
 
 #define COMPAT_USER_HZ		100
 #define COMPAT_UTS_MACHINE	"armv8l\0\0"