build: allow parser.c to be created if missing

during regen-pegen

Previously, `regen-pegen` would fail if Parser/parser.c was deleted
(e.g. due to merge conflicts), requiring a manual `touch` to proceed.
This change passes `--create` to update_file.py to allow the file
to be created automatically if it doesn't exist.

Author: dura0ok

Makefile.pre.in

@@ -1948,7 +1948,7 @@ regen-pegen:
 		$(srcdir)/Grammar/python.gram \
 		$(srcdir)/Grammar/Tokens \
 		-o $(srcdir)/Parser/parser.c.new
-	$(UPDATE_FILE) $(srcdir)/Parser/parser.c $(srcdir)/Parser/parser.c.new
+	$(UPDATE_FILE) --create $(srcdir)/Parser/parser.c $(srcdir)/Parser/parser.c.new
 
 .PHONY: regen-ast
 regen-ast: