Include boost/thread.hpp in validation.cpp

Summary: It is required for windows build to find boost::this_thread::interruption_point . Not sure why it is found at all on other plateforms.

Test Plan:

cmake -GNinja .. -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Win32.cmake -DBUILD_BITCOIN_SEEDER=Off
ninja

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1788