silence the compiler.  it doesn't know that lockinit will set
arch to R4K if nothing else can be found.

Reference: /n/sources/patch/applied/mipslock
Date: Mon Nov 28 22:06:48 CET 2011
Signed-off-by: quanstro@quanstro.net

--- /sys/src/libc/mips/lock.c	Mon Nov 28 22:05:42 2011
+++ /sys/src/libc/mips/lock.c	Mon Nov 28 22:05:41 2011
@@ -107,6 +107,7 @@
 			return 0;
 		return 1;
 	case R4K:
+	default:		/* silence compiler */
 		if(C_4ktas(&lk->val))
 			return 0;
 		return 1;
@@ -149,6 +150,7 @@
 	case MAGNUMII:
 		return C_3ktas(p);
 	case R4K:
+	default:		/* silence compiler */
 		return C_4ktas(p);
 	case POWER:
 		/* Use low order lock bits to generate hash */