pygtk: fix pygobject-codegen-2.0 path

Closes Homebrew/homebrew#29330.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/pygtk.rb

@@ -27,5 +27,11 @@ class Pygtk < Formula
     # pygtk to pygobject. But our pkgfiles point into the cellar and in the
     # pygtk-cellar there is no pygobject.
     inreplace lib/'pkgconfig/pygtk-2.0.pc', 'codegendir=${datadir}/pygobject/2.0/codegen', "codegendir=#{HOMEBREW_PREFIX}/share/pygobject/2.0/codegen"
+    inreplace bin/"pygtk-codegen-2.0", "exec_prefix=${prefix}", "exec_prefix=#{Formula["pygobject"].opt_prefix}"
+  end
+
+  test do
+    (testpath/"codegen.def").write("(define-enum asdf)")
+    system "#{bin}/pygtk-codegen-2.0", "codegen.def"
   end
 end