genxml: use the isalpha method rather than str.isalpha.

This fixes gen_pack_header to work on python 2, where name[0] is unicode
not str.

Signed-off-by: Dylan Bake <dylanx.c.baker@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
cc: 12.0 <mesa-stable@lists.freedesktop.org>

src/intel/genxml/gen_pack_header.py

@@ -201,7 +201,7 @@ def to_alphanum(name):
 
 def safe_name(name):
     name = to_alphanum(name)
-    if not str.isalpha(name[0]):
+    if not name[0].isalpha():
         name = '_' + name
 
     return name