Add missing <functional> include [blocks: #6749]

[tautschnig]

Headers should include what they, and not rely on nonstd/optional to
provide the header.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6764 (dea6182) into develop (57a8097) will increase coverage by 0.00%.
The diff coverage is 77.77%.

@@           Coverage Diff            @@
##           develop    #6764   +/-   ##
========================================
  Coverage    76.89%   76.90%           
========================================
  Files         1590     1590           
  Lines       183979   183963   -16     
========================================
- Hits        141478   141477    -1     
+ Misses       42501    42486   -15     

Impacted Files	Coverage Δ
...o-instrument/accelerate/polynomial_accelerator.cpp	0.00% <0.00%> (ø)
...oto-instrument/accelerate/polynomial_accelerator.h	100.00% <ø> (ø)
src/goto-programs/resolve_inherited_component.h	100.00% <ø> (ø)
src/solvers/flattening/boolbv.h	62.50% <ø> (ø)
src/solvers/flattening/boolbv_map.h	100.00% <ø> (ø)
src/util/expr.h	97.22% <ø> (ø)
src/solvers/flattening/boolbv_overflow.cpp	48.68% <76.00%> (-1.91%)	⬇️
src/solvers/flattening/boolbv.cpp	80.39% <80.00%> (-0.18%)	⬇️
src/analyses/goto_rw.cpp	63.28% <100.00%> (+0.45%)	⬆️
src/util/simplify_expr_array.cpp	90.72% <100.00%> (+0.19%)	⬆️
... and 1 more

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