um: Remove duplicated include in syscalls_64.c

Fix following includecheck warning:
./arch/x86/um/syscalls_64.c: registers.h is included more than once.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Fixes: dbba7f70

 ("um: stop polluting the namespace with registers.h contents")
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Richard Weinberger <richard@nod.at>