headers_check fix: x86, prctl.h

fix the following 'make headers_check' warning:

  usr/include/asm/prctl.h:10: extern's make no sense in userspace

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
diff --git a/arch/x86/include/asm/syscalls.h b/arch/x86/include/asm/syscalls.h
index c0b0bda..e26d34b 100644
--- a/arch/x86/include/asm/syscalls.h
+++ b/arch/x86/include/asm/syscalls.h
@@ -74,6 +74,7 @@
 asmlinkage long sys_execve(char __user *, char __user * __user *,
 			   char __user * __user *,
 			   struct pt_regs *);
+long sys_arch_prctl(int, unsigned long);
 
 /* kernel/ioport.c */
 asmlinkage long sys_iopl(unsigned int, struct pt_regs *);