g15helper: update G15 SDK paths for OS X.

The g15helper program is currently coded against Logitech's
legacy G15 SDK. Logitech's newer SDK, which is the only one
shipped in their newer versions of Logitech Gaming Software,
is not compatible.

For OS X, the legacy SDK is still available for download
if you download the version of LGS (GamePanel 3.x) that
supports OS X 10.4 through 10.6. However, the SDK shipped
in this version of their software uses slightly different
paths than we expect.

This commit changes the paths to make sense with the SDK
shipped with the GamePanel 3.x software.