add note about php_url struct changes

Author: remicollet

UPGRADING.INTERNALS

@@ -18,6 +18,7 @@ PHP 7.3 INTERNALS UPGRADE NOTES
   o. IS_UNUSED
   p. VM instruction operands (FETCH_CLASS, FETCH_CONSTANT, CATCH)
   q. sapi_cli_single_write()
+  r. php_url
 
 2. Build system changes
   a. Unix build system changes
@@ -126,6 +127,8 @@ PHP 7.3 INTERNALS UPGRADE NOTES
 
  q.  sapi_cli_single_write() now returns ssize_t instead of size_t.
 
+ r.  fields of php_url struct change from char * to zend_string *
+
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 2. Build system changes
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