I recently built a Kestral 200 with internal brake routing. I was wondering if any type of sealant is put around the cable entry holes for a water seal or is it usually not a problem?

Not a problem, particularly with a carbon frame. It won't corrode if a little water gets in and the water should drain to the bottom bracket eventually. Trek used internal cable routing on some of their late-'80's and early-'90's Al frames with no seals at the entry or exit holes and no harm came from it.

Thanks

I beg to differ slightly. While carbon frames themselves are virtually immune to corrosion or water damage, the parts threaded into them aren't, especially alloy BB cups. As HillRider correctly states, water will drain to the BB shell, which is sort of the bilge of bike frames. What happens next varies bike to bike, and affects frames of all materials.

If there's a weep hole the BB will drain and you'll have little or no problems, if not water will work into the BB threads and cause nightmarish corrosion problems, in extreme cases fusing the cups into the frame.

Regardless of frame material, don't worry about sealing, focus instead on providing good drainage, and use best practices including anti-seize when installing bottom brackets.