[Bug] minor compilation error in demo_colored_registration.cpp

[sethgi]

Hello,

This repo is great!

One very minor bug - when building with demos enabled but tests disabled, gtest may not be available. As a result, line 5 of demo_colored_registration.cpp results in "No such file or directory" for <gtest/getst.h>

Simply removing this line allows the demos to build properly.

Thanks,
Seth