cleanup: remove 4-byte hole in struct schema_field_value

Alignment of structure members to "natural" address boundaries will
cause to a 4-byte gap between field_name and field_name_len on
64-bit platform. If the members will be swapped, the gap is gone.

src/tarantool_schema.h

@@ -15,8 +15,8 @@ enum field_type {
 
 struct schema_field_value {
 	uint32_t        field_number;
-	char           *field_name;
 	uint32_t        field_name_len;
+	char           *field_name;
 	enum field_type field_type;
 };