Add missing include for std::map

[tautschnig]

The file relies on irep.h providing std::map, which it does not do if
setting NAMED_SUB_IS_FORWARD_LIST.

• 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 #5556 (2732bdb) into develop (27f2a8a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5556   +/-   ##
========================================
  Coverage    68.53%   68.53%           
========================================
  Files         1187     1187           
  Lines        98263    98263           
========================================
  Hits         67347    67347           
  Misses       30916    30916           

Flag	Coverage Δ
cproversmt2	43.09% <ø> (ø)
regression	65.71% <ø> (ø)
unit	32.26% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/util/irep_hash_container.h	100.00% <ø> (ø)

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