Remove harmless warning on win32
diff --git a/main.c b/main.c
index 3fb380e..68e8dab 100644
--- a/main.c
+++ b/main.c
@@ -263,7 +263,10 @@ static char datestamp[40];
static char blocktime[30];
static char *opt_kernel = NULL;
-static char *opt_stderr_cmd = NULL;
+
+#if defined(unix)
+ static char *opt_stderr_cmd = NULL;
+#endif // defined(unix)
enum cl_kernel chosen_kernel;