um/arch_prctl: Fix fallout from x86 arch_prctl() rework

The recent arch_prctl rework added a bracket instead of a comma. Fix it.

Fixes: 17a6e1b8e8e8 ("x86/arch_prctl/64: Rename do_arch_prctl() to do_arch_prctl_64()")
Signed-off-by: Kyle Huey <khuey@kylehuey.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: kbuild-all@01.org
Link: http://lkml.kernel.org/r/20170320230535.11281-1-khuey@kylehuey.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/um/syscalls_64.c

@@ -12,7 +12,7 @@
 #include <asm/prctl.h> /* XXX This should get the constants from libc */
 #include <os.h>
 
-long arch_prctl(struct task_struct *task, int option)
+long arch_prctl(struct task_struct *task, int option,
 		unsigned long __user *arg2)
 {
 	unsigned long *ptr = arg2, tmp;