go: don't patch HEAD installs

The patch was inserting an extra case statement, but a duplicate case
statement is already included in HEAD (and devel).

Closes Homebrew/legacy-homebrew#23787.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/go.rb

@@ -34,7 +34,7 @@ class Go < Formula
   # Upstream patch for a switch statement that causes a clang error
   # Should be in the next release.
   # http://code.google.com/p/go/source/detail?r=000ecca1178d67c9b482d3fb0b6a1bc4aeef2472&path=/src/cmd/ld/lib.c
-  def patches; DATA; end unless build.devel?
+  def patches; DATA; end unless build.devel? or build.head?
 
   def install
     # For Clang cgo support Go needs to be able to tell through CC.