template: add doc.go to Makefile

template: add doc.go to Makefile

The template package is the only one that has a doc.go not mentioned
in its Makefile.

This doesn't seem to bother godoc, but seems like a bug to me.

$ for d in $(find pkg -name doc.go); do echo $d; grep doc.go $(dirname $d)/Makefile; done
pkg/fmt/doc.go
        doc.go\
pkg/go/doc/doc.go
        doc.go\
pkg/gob/doc.go
        doc.go\
pkg/html/doc.go
        doc.go\
pkg/old/template/doc.go
        doc.go\
pkg/sync/atomic/doc.go
        doc.go\
pkg/template/doc.go

R=r
CC=golang-dev
https://golang.org/cl/5003047