src/svgtiny_strings.h: intern a "userdata" key string

To implement libcss's select handler API, we need to implement two
functions that get/set "userdata" on a node. The corresponding libdom
API allows you to set arbitrary data in the form of key->value pairs;
so to identify THIS particular data, we need to use the same string
each time. For that reason we intern "_libcss_user_data" and call it
the "userdata_key".