cmd/compile: add new format to known_formats

This change fixes a TestFormat failure in fmt_test by adding a
recently introduced new known format (%q for syntax.Error).

Fixes #28621

Change-Id: I026ec88c334549a957a692c1652a860c57e23dae
Reviewed-on: https://go-review.googlesource.com/c/147837Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/fmt_test.go

@@ -660,6 +660,7 @@ var knownFormats = map[string]string{
 	"cmd/compile/internal/ssa.rbrank %d":              "",
 	"cmd/compile/internal/ssa.regMask %d":             "",
 	"cmd/compile/internal/ssa.register %d":            "",
+	"cmd/compile/internal/syntax.Error %q":            "",
 	"cmd/compile/internal/syntax.Expr %#v":            "",
 	"cmd/compile/internal/syntax.Node %T":             "",
 	"cmd/compile/internal/syntax.Operator %s":         "",