biology/gcta: Use bundled version of eigen3

... in preparation to upgrading math/eigen to 3.4.0.

This version of gcta (1.26.0) doesn't support eigen-3.4.0, and
it can't be upgraded because latest versions of gcta use
Intel's closed source MKL library.

PR: 265032
Suggested by: tcberner@
Approved by: portmgr (interoperability)