Fix quadmath linking error

[shivupa]

I believe this is needed because as I understand it,

• when quadmath is found this function is defined

but

• because it is not defined in src/rys_roots.h, then this won't have a symbol in the resulting shared library so I was getting a linking error

This matches pow_ and powl_ so I assume it is correct, but feel free to close if this is not needed.

This does fix the error with gcc/10.2.0 (unfortunately the newest on this local dev machine that I was using.

edit: similar to #120