python: For Xcode-only, add the the Tk header dir

Closes Homebrew/homebrew#15351.

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

Formula/python.rb

@@ -191,6 +191,8 @@ class Python < Formula
         # Help Python's build system (distribute/pip) to build things on Xcode-only systems
         # The setup.py looks at "-isysroot" to get the sysroot (and not at --sysroot)
         cflags += " -isysroot #{MacOS.sdk_path}"
+        # For the Xlib.h, Python needs this header dir
+        cflags += " -I#{MacOS.sdk_path}/System/Library/Frameworks/Tk.framework/Versions/8.5/Headers"
         ldflags += " -isysroot #{MacOS.sdk_path}"
         # Same zlib.h-not-found-bug as in env :std (see below)
         args << "CPPFLAGS=-I#{MacOS.sdk_path}/usr/include"