quote.h: simplify the inclusion

Attempting to include quote.h without first including strbuf.h results
in warnings:

 ./quote.h:33:33: warning: ‘struct strbuf’ declared inside parameter list
 ./quote.h:33:33: warning: its scope is only this definition or declaration, which is probably not what you want
 ./quote.h:34:34: warning: ‘struct strbuf’ declared inside parameter list
 ...

Add a toplevel declaration for struct strbuf to avoid this.

While at it, stop including system headers from quote.h.  git source
files already need to include git-compat-util.h sooner to ensure the
appropriate feature test macros are defined.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

quote.h

@@ -1,8 +1,7 @@
 #ifndef QUOTE_H
 #define QUOTE_H
 
-#include <stddef.h>
-#include <stdio.h>
+struct strbuf;
 
 /* Help to copy the thing properly quoted for the shell safety.
  * any single quote is replaced with '\'', any exclamation point