libgaminggear: init at 0.15.1

This is a requirement for roccat-tools, which is going to be introduced
soon.

The reason why I'm using propagatedBuildInputs here is because the
pkg-config file lists *all* of the dependencies in Requires and
Requires.private, so those libraries are needed whenever any software
uses that library.

Signed-off-by: aszlig <aszlig@nix.build>