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.

src/ctx.c

@@ -63,6 +63,8 @@ static xmpp_mem_t xmpp_default_mem = {
     realloc
 };
 
+static const char * const xmpp_log_level_name[4] = {"DEBUG", "INFO", "WARN", "ERROR"};
+
 void xmpp_default_logger(void * const userdata,
 			 const xmpp_log_level_t level,
 			 const char * const area,