synergy: Fix support for XRandR again.

Regression introduced by f91dacdd.

Accidentally thought that it's compiling with XRandR support enabled,
because the cmake output said so:

Looking for XRRQueryExtension in Xrandr - found

Unfortunately, despite this message, the relevant part is:

Looking for XRRNotifyEvent - not found

So, ea4afb73

 still holds true and I've added a small comment to avoid
this from happening in the future.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>