super: migrate xml2 cmake to mac/super

Looks like it was accidentally left behind (?)

Library/Homebrew/extend/ENV/super.rb

@@ -200,7 +200,6 @@ module Superenv
 
   def determine_cmake_include_path
     paths = []
-    paths << "#{effective_sysroot}/usr/include/libxml2" unless deps.any? { |d| d.name == "libxml2" }
     paths += homebrew_extra_cmake_include_paths
     paths.to_path_s
   end

Library/Homebrew/extend/os/mac/extend/ENV/super.rb

@@ -55,6 +55,7 @@ module Superenv
 
   def homebrew_extra_cmake_include_paths
     paths = []
+    paths << "#{effective_sysroot}/usr/include/libxml2" unless deps.any? { |d| d.name == "libxml2" }
     paths << "#{effective_sysroot}/usr/include/apache2" if MacOS::Xcode.without_clt?
     paths << MacOS::X11.include.to_s << "#{MacOS::X11.include}/freetype2" if x11?
     paths << "#{effective_sysroot}/System/Library/Frameworks/OpenGL.framework/Versions/Current/Headers"