Adhere to PHP semantics with g:phpcomplete_relax_static_constraint on

When calling a non-static class method as a static method, php will issue an
E_STRICT level warning. If you do this with a property, that's a fatal error.
Therefore the completion shouldn't show the non-static properties even if
g:phpcomplete_relax_static_constraint is enabled.

Example:
<?php
class C {
  public $foo = 'bar';
  public function foo() {
    return 'bar';
  }
}
var_dump(C::foo()); // E_STRICT warning
var_dump(C::$foo); // Fatal error