[dev.ssa] cmd/compile/internal/gc: fix stringsym call

Forgot to add this in the tip merge.

Change-Id: I0e5a2681133f4ae7a7c360ae2c2d71d46420c693
Reviewed-on: https://go-review.googlesource.com/11793Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/ssa.go

@@ -1321,7 +1321,8 @@ type ssaExport struct {
 // is a global string constant containing s.
 func (*ssaExport) StringSym(s string) interface{} {
 	// TODO: is idealstring correct?  It might not matter...
-	return &ssa.ExternSymbol{Typ: idealstring, Sym: stringsym(s)}
+	hdr, _ := stringsym(s)
+	return &ssa.ExternSymbol{Typ: idealstring, Sym: hdr}
 }
 
 // Log logs a message from the compiler.