meson option requires config.h for meson options to operate properly

This was removed in
https://github.com/openbmc/phosphor-pid-control/commit/f8b6e55

Change-Id: I42d21365516a104cc0372d1fa72477642784c4d1
Signed-off-by: Pete O_o <pmb@meta.com>
diff --git a/test/pid_fancontroller_unittest.cpp b/test/pid_fancontroller_unittest.cpp
index 315931f..3ee585e 100644
--- a/test/pid_fancontroller_unittest.cpp
+++ b/test/pid_fancontroller_unittest.cpp
@@ -1,3 +1,4 @@
+#include "config.h"
 
 #include "pid/ec/pid.hpp"
 #include "pid/fan.hpp"