firmware: silent gcc 6.1 warning

Add unused attribute to potentially unused variable.

Change-Id: I3a4ef3919692cbd9d3041f718117be912afac03d
Signed-off-by: Antonio Borneo <[email protected]>
diff --git a/firmware/os/drivers/vsync/vsync.c b/firmware/os/drivers/vsync/vsync.c
index 1a250fc..68147b0 100644
--- a/firmware/os/drivers/vsync/vsync.c
+++ b/firmware/os/drivers/vsync/vsync.c
@@ -61,7 +61,7 @@
         } \
     } while (0);
 
-static const bool enable_debug = 0;
+static const bool __attribute__((unused)) enable_debug = 0;
 
 static struct SensorTask
 {