genxml: use PYTHON3

Allows the build to work when the python3 binary is not "python3".

v2: remove x bit from the script at Emil's suggestion

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/intel/genxml/Makefile.am

@@ -26,7 +26,9 @@ BUILT_SOURCES =                                         \
 	gen8_pack.h					\
 	gen9_pack.h
 
+PYTHON3_GEN = $(AM_V_GEN)$(PYTHON3) $(PYTHON_FLAGS)
+
 %_pack.h : %.xml gen_pack_header.py
-	$(AM_V_GEN) $(srcdir)/gen_pack_header.py $< > $@
+	$(PYTHON3_GEN) $(srcdir)/gen_pack_header.py $< > $@
 
 CLEANFILES = $(BUILT_SOURCES)