MIPS: Fix randconfig build error.

  CC      arch/mips/kernel/ptrace.o
In file included from arch/mips/kernel/ptrace.c:42:0:
arch/mips/kernel/ptrace.c: In function ‘mips_get_syscall_arg’:
/home/ralf/src/linux/linux-mips/arch/mips/include/asm/syscall.h:60:1: error: control reaches end of non-void function [-Werror=return-type]
cc1: all warnings being treated as errors
make[2]: *** [arch/mips/kernel/ptrace.o] Error 1
make[1]: *** [arch/mips/kernel] Error 2
make: *** [arch/mips] Error 2

Fixed by marking the end of mips_get_syscall_arg() as unreachable.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/syscall.h

@@ -13,6 +13,7 @@
 #ifndef __ASM_MIPS_SYSCALL_H
 #define __ASM_MIPS_SYSCALL_H
 
+#include <linux/compiler.h>
 #include <linux/audit.h>
 #include <linux/elf-em.h>
 #include <linux/kernel.h>
@@ -57,6 +58,8 @@ static inline unsigned long mips_get_syscall_arg(unsigned long *arg,
 	default:
 		BUG();
 	}
+
+	unreachable();
 }
 
 static inline long syscall_get_return_value(struct task_struct *task,