runtime: add timespec definition for freebsd

R=golang-dev, devon.odell
CC=golang-dev
https://golang.org/cl/5340058
diff --git a/src/pkg/runtime/freebsd/amd64/defs.h b/src/pkg/runtime/freebsd/amd64/defs.h
index b101b19..8a222dc 100644
--- a/src/pkg/runtime/freebsd/amd64/defs.h
+++ b/src/pkg/runtime/freebsd/amd64/defs.h
@@ -184,6 +184,12 @@
 	byte pad_godefs_0[12];
 };
 
+typedef struct Timespec Timespec;
+struct Timespec {
+	int64 tv_sec;
+	int64 tv_nsec;
+};
+
 typedef struct Timeval Timeval;
 struct Timeval {
 	int64 tv_sec;