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.

strophe.h

@@ -63,8 +63,6 @@ typedef enum {
   XMPP_LEVEL_ERROR
 } xmpp_log_level_t;
 
-static const char * const xmpp_log_level_name[4] = {"DEBUG", "INFO", "WARN", "ERROR"};
-
 typedef enum {
     XMPP_UNKNOWN,
     XMPP_CLIENT,