add missing unordered_map include

Summary: This was missed in D17217 and it seems to cause compiler errors on some systems (native OSX)

Test Plan:
ninja all check-all

After landing check that the CI build is fixed for Native OSX

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17252