astro/stellarium: unbreak the port's build (after commit d7c13b0bf44f)

Requesting specific version of `math/fast_float' port, even the major
one, is not reliable because it bumps too frequently; stop asking for
any version and assume that whatever we have in the ports should work.

PR: 284879
Submitted by: Frank Reisert
Reported by: pkg-fallout