Forward definitions to allow stand-alone parsing

Since https_internal.h is only intended to be included by other http/
files, this isn't really a mistake.  However, there's no harm in adding
these forward definitions.

http/https_internal.h

@@ -1,6 +1,13 @@
 #ifndef HTTPS_INTERNAL_H_
 #define HTTPS_INTERNAL_H_
 
+#include <stddef.h>
+
+/* Forward definitions. */
+struct http_request;
+struct http_response;
+struct sock_addr;
+
 /*
  * Function pointers defined in http.c; we set them from https_request in
  * order to avoid requiring unencrypted HTTP code to link to libssl.