cmake: Use INSTALL(CODE) instead of add_custom_target to create kinfocenter symlink

Targets created via add_custom_target are always out-of-date and will
always run on every build, slowing down the overall build and creating
noise in build logs. Instead, we can use INSTALL(CODE) to create the
necessary symlink at install time directly without using an intermediate
target.

End result is that when building kinfocenter again after an initial
build, we now get "ninja: no work to do." whereas before, the symlink
target would be rebuilt every time.