patch 8.1.2394: using old C style comments

Problem:    Using old C style comments.
Solution:   Use // comments where appropriate.
diff --git a/src/quickfix.c b/src/quickfix.c
index 72cdc6e..3d12374 100644
--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -7880,7 +7880,7 @@
 	    curwin->w_llist = qi;
     }
 }
-#endif /* FEAT_QUICKFIX */
+#endif // FEAT_QUICKFIX
 
 #if defined(FEAT_EVAL) || defined(PROTO)
 # ifdef FEAT_QUICKFIX