Don't install libsla.so

[gijzelaerr]

as reported here: https://github.com/UCBerkeleySETI/bl_packaging/issues/1
to quote @david-macmahon:

The thing is that they offer the same functionality: positional astronomy routines. Basically gpuspec (gbt-seti) copied this file from presto so it wouldn't have to depend on presto, but it was never intended to be a fork. It might be better, for now, to remove it from gpuspec and make gbt-seti depend on presto.

[gijzelaerr]

implemented in PR #8