cmd/cc/cc.h: Add a #pragma for %S used (only) in cmd/cc/sub.c.

Eliminates a format consistency warning.

R=gloang-dev, r
CC=golang-dev
https://golang.org/cl/8217043

src/cmd/cc/cc.h

@@ -800,6 +800,7 @@ int	machcap(Node*);
 #pragma	varargck	type	"Q"	int32
 #pragma	varargck	type	"O"	int
 #pragma	varargck	type	"O"	uint
+#pragma	varargck	type	"S"	ushort*
 #pragma	varargck	type	"T"	Type*
 #pragma	varargck	type	"U"	char*
 #pragma	varargck	type	"|"	int