cmake: Make lgc's interface directory public
cmake: Make lgc's interface directory public

Change-Id: I6ccb5230f2c357b1c63d1e96981f56ed331354a3

GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,googlestadia/llpc,googlestadia/llpc