Add gsl include directory at compilation

[mtaillefumier]

cmake searches for GSL but the target is used nowhere leading to a compilation error. GSL::gsl should be added to sirius_cxx to fix the potential issue

[simonpintarelli]

lgtm
I wonder why it was linking gsl correctly in spack?

I double checked that it doesn't link gsl cblas. In the current cmake this is a separate target, iirc this wasn't always the case.