go/build: use back quotes for regular expression

R=adg
CC=golang-dev
https://golang.org/cl/4643045
diff --git a/src/pkg/go/build/build.go b/src/pkg/go/build/build.go
index d880746..5854892 100644
--- a/src/pkg/go/build/build.go
+++ b/src/pkg/go/build/build.go
@@ -348,7 +348,7 @@
 	return append(a, args...)
 }
 
-var cgoRe = regexp.MustCompile("[/\\:]")
+var cgoRe = regexp.MustCompile(`[/\\:]`)
 
 func (b *build) cgo(cgofiles []string) (outGo, outObj []string) {
 	// cgo