runtime: silence warnings

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5656084

src/pkg/runtime/signal_plan9_386.c

@@ -13,6 +13,7 @@ runtime·signame(int32)
 void
 runtime·sigenable(uint32 sig)
 {
+	USED(sig);
 }
 
 void

src/pkg/runtime/signal_windows_386.c

@@ -83,6 +83,7 @@ runtime·sighandler(ExceptionRecord *info, Context *r, G *gp)
 void
 runtime·sigenable(uint32 sig)
 {
+	USED(sig);
 }
 
 void

src/pkg/runtime/signal_windows_amd64.c

@@ -90,6 +90,7 @@ runtime·sighandler(ExceptionRecord *info, Context *r, G *gp)
 void
 runtime·sigenable(uint32 sig)
 {
+	USED(sig);
 }
 
 void