com_err: Constify xyz_error_strings arrays.

Note: This changes the types of public symbols.  It is unlikely that
any applications would rightly _write_ to these arrays, but it is
possible they might require some UNCONST in order to pass the
pointers to other functions that are missing const qualifiers.

lib/com_err/com_err.c

@@ -63,7 +63,7 @@ error_message (long code)
 }
 
 KRB5_LIB_FUNCTION int KRB5_LIB_CALL
-init_error_table(const char **msgs, long base, int count)
+init_error_table(const char *const *msgs, long base, int count)
 {
     initialize_error_table_r(&_et_list, msgs, count, base);
     return 0;