style: fix includes

* src/fixits.c: Follow our usual pattern.
* src/scan-code.l, src/scan-gram.l, src/scan-skel.l: Prefer "" to
include src/ headers.
* README-hacking.md: Document the pattern.

src/print-xml.h

@@ -21,8 +21,6 @@
 #ifndef PRINT_XML_H_
 # define PRINT_XML_H_
 
-# include <config.h>
-
 # include <stdio.h>
 
 void xml_indent (FILE *out, int level);