patch 8.1.0944: format of nbdbg() arguments is not checked

Problem:    Format of nbdbg() arguments is not checked.
Solution:   Add format attribute.  Fix reported problems. (Dominique Pelle,
            closes #3992)
diff --git a/src/nbdebug.h b/src/nbdebug.h
index 6331f49..16dd847 100644
--- a/src/nbdebug.h
+++ b/src/nbdebug.h
@@ -42,7 +42,11 @@
 } WtWait;
 
 
-void		 nbdbg(char *, ...);
+void		 nbdbg(char *, ...)
+#ifdef USE_PRINTF_FORMAT_ATTRIBUTE
+   __attribute__((format(printf, 1, 2)))
+#endif
+;
 
 void nbdebug_wait(u_int wait_flags, char *wait_var, u_int wait_secs);
 void nbdebug_log_init(char *log_var, char *level_var);