cmd/go: fix including of _cgo_export.h

This will add the temporary object directory into the lookup
path so that cgo-exported function declarations may be
included from C files.

This was previously applied by CL 5600043, and apparently
removed by mistake on CL 5598045.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5610054

src/cmd/go/build.go

@@ -1148,6 +1148,9 @@ func (b *builder) cgo(p *Package, cgoExe, obj string, gccfiles []string) (outGo,
 		}
 	}
 
+	// Allows including _cgo_export.h from .[ch] files in the package.
+	cgoCFLAGS = append(cgoCFLAGS, "-I", obj)
+
 	// cgo
 	// TODO: CGOPKGPATH, CGO_FLAGS?
 	gofiles := []string{obj + "_cgo_gotypes.go"}