[dev.cc] runtime: fix linux build

TBR=austin
CC=golang-codereviews
https://golang.org/cl/176760044
diff --git a/src/runtime/defs_linux_arm.go b/src/runtime/defs_linux_arm.go
index 1e7c679..7f83002 100644
--- a/src/runtime/defs_linux_arm.go
+++ b/src/runtime/defs_linux_arm.go
@@ -88,6 +88,10 @@
 	ts.tv_sec = x
 }
 
+func (ts *timespec) set_nsec(x int32) {
+	ts.tv_nsec = x
+}
+
 type sigaltstackt struct {
 	ss_sp    *byte
 	ss_flags int32