genxml: mark gen_pack_header.py as encoded in utf-8

There is unicode in this file, and I'm actually surprised that the
python interpreter hasn't gotten grumpy.

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
cc: 12.0 <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 9f50e3572c)

src/intel/genxml/gen_pack_header.py

@@ -1,4 +1,5 @@
 #!/usr/bin/env python3
+#encoding=utf-8
 
 import xml.parsers.expat
 import re