clang: make clang-version-inc.py request python2 interpreter

Test: run script in Fedora 31 with (default) python3 interpreter.

```
$ env python --version
Python 3.7.9
```

Change-Id: I993896d865131f2d5a6688357eeba554796d5b60
diff --git a/clang-version-inc.py b/clang-version-inc.py
index eeffb62..d55d25f 100755
--- a/clang-version-inc.py
+++ b/clang-version-inc.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2
 
 import os
 import sys