genxml: add generated headers to EXTRA_DIST

Building the Mesa 12.0.3 distfile failed on a system without python
as generated files were not included in the distfile.

Cc: "12.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

src/intel/Makefile.genxml.am

@@ -23,6 +23,10 @@ BUILT_SOURCES += \
 	$(GENXML_GENERATED_FILES) \
 	$(AUBINATOR_GENERATED_FILES)
 
+EXTRA_DIST += \
+	$(GENXML_GENERATED_FILES) \
+	$(AUBINATOR_GENERATED_FILES)
+
 SUFFIXES = _pack.h _xml.h .xml
 
 $(GENXML_GENERATED_FILES): genxml/gen_pack_header.py