patch 8.1.2394: using old C style comments

Problem:    Using old C style comments.
Solution:   Use // comments where appropriate.
diff --git a/src/sign.c b/src/sign.c
index 71f21b0..935ec3c 100644
--- a/src/sign.c
+++ b/src/sign.c
@@ -701,8 +701,8 @@
 
     return count;
 }
-#  endif /* FEAT_SIGN_ICONS */
-# endif /* FEAT_NETBEANS_INTG */
+#  endif // FEAT_SIGN_ICONS
+# endif // FEAT_NETBEANS_INTG
 
 /*
  * Delete signs in group 'group' in buffer "buf". If 'group' is '*', then
@@ -2744,4 +2744,4 @@
     }
 }
 
-#endif /* FEAT_SIGNS */
+#endif // FEAT_SIGNS