qapi: Fix code generated for optional conditional struct member

The generated member visit neglects to emit #if around a conditional
struct member's has_ variable.  For instance,
tests/qapi-schema/qapi-schema-test.json generates

    #if defined(TEST_IF_STRUCT)
    bool visit_type_TestIfStruct_members(Visitor *v, TestIfStruct *obj, Error **errp)
    {
--->	bool has_baz = !!obj->baz;

	if (!visit_type_int(v, "foo", &obj->foo, errp)) {
	    return false;
	}
    #if defined(TEST_IF_STRUCT_MEMBER)
	if (!visit_type_int(v, "bar", &obj->bar, errp)) {
	    return false;
	}
    #endif /* defined(TEST_IF_STRUCT_MEMBER) */
    #if defined(TEST_IF_STRUCT_MEMBER)
	if (visit_optional(v, "baz", &has_baz)) {
	    if (!visit_type_str(v, "baz", &obj->baz, errp)) {
		return false;
	    }
	}
    #endif /* defined(TEST_IF_STRUCT_MEMBER) */
	return true;
    }
    [...]
    #endif /* defined(TEST_IF_STRUCT) */

Won't compile when TEST_IF_STRUCT is defined and TEST_IF_STRUCT_MEMBER
isn't.

Fix that the obvious way:

    #if defined(TEST_IF_STRUCT_MEMBER)
	bool has_baz = !!obj->baz;
    #endif /* defined(TEST_IF_STRUCT_MEMBER) */

Fixes: 44ea9d9be33c (qapi: Start to elide redundant has_FOO in generated C)
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230316071325.492471-14-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

scripts/qapi/visit.py

@@ -74,11 +74,13 @@ def gen_visit_object_members(name: str,
     sep = ''
     for memb in members:
         if memb.optional and not memb.need_has():
+            ret += memb.ifcond.gen_if()
             ret += mcgen('''
     bool has_%(c_name)s = !!obj->%(c_name)s;
 ''',
                          c_name=c_name(memb.name))
             sep = '\n'
+            ret += memb.ifcond.gen_endif()
     ret += sep
 
     if base: