add format attribute to set_dbparam

src/tcbtdb.c:124:49: error: format string is not a string literal
      [-Werror,-Wformat-nonliteral]
  n = vsnprintf (params + len, DB_PARAMS - len, fmt, args);
                                                ^~~
1 error generated.

src/tcbtdb.c

@@ -114,6 +114,9 @@ tc_db_rmdir ()
  * On error, a negative number is returned.
  * On success, the number of characters that would have been written is
  * returned. */
+#if defined(__GNUC__) || defined(__clang__)
+__attribute__((__format__(printf,3,4)))
+#endif
 static int
 set_dbparam (char *params, int len, const char *fmt, ...)
 {