Move the log_level_name[] definitions back to private status within the library.

Defining static data in the header causes warnings on Apple gcc, and in general
this should be the client's responsibility. The names are obvious from the
log_level enum, which is public, and it's only one line of boilerplate to
copy in implementing a custom logging function.

examples/basic_logging.c

@@ -21,7 +21,9 @@ void log_handler(void * const userdata,
 		 const char * const area,
 		 const char * const msg)
 {
-    fprintf(stderr, "%s %s %s\n", area, xmpp_log_level_name[level], msg);
+    static const char * const log_level_name[4] = {"DEBUG", "INFO", "WARN", "ERROR"};
+    
+    fprintf(stderr, "%s %s %s\n", area, log_level_name[level], msg);
 }
 
 void conn_handler(xmpp_conn_t * const conn, const xmpp_conn_event_t status,