patch 9.0.0859: compiler warning for unused variable

Problem:    Compiler warning for unused variable.
Solution:   Add #ifdef.
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index a445c31..02bca38 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -4308,8 +4308,9 @@
     lnum = MAXLNUM;
     do
     {
+#ifdef FEAT_FOLDING
 	int base_char = *cmd;
-
+#endif
 	switch (*cmd)
 	{
 	    case '.':			    // '.' - Cursor position