patch 8.0.0425: build errors when building without folding

Problem:    Build errors when building without folding.
Solution:   Add #ifdefs. (John Marriott)
diff --git a/src/syntax.c b/src/syntax.c
index 0d9cd2c..11f679c 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -1183,11 +1183,12 @@
     void
 syn_stack_free_all(synblock_T *block)
 {
+#ifdef FEAT_FOLDING
     win_T	*wp;
+#endif
 
     syn_stack_free_block(block);
 
-
 #ifdef FEAT_FOLDING
     /* When using "syntax" fold method, must update all folds. */
     FOR_ALL_WINDOWS(wp)