Add missing include file [blocks: #4056]

[tautschnig]

We will enable proper type checking when linking library symbols, which
(rightly) fails with a missing include file.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 3f2d633).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/102643083