add missing narrow.h includes

[kroening]

This adds missing includes for util/narrow.h in two files that use narrow_cast<...>.

• 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/
• n/a 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 #7113 (65784b5) into develop (8b7b7a9) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #7113      +/-   ##
===========================================
- Coverage    77.92%   77.92%   -0.01%     
===========================================
  Files         1576     1576              
  Lines       181645   181644       -1     
===========================================
- Hits        141541   141540       -1     
  Misses       40104    40104              

Impacted Files	Coverage Δ
src/solvers/lowering/byte_operators.cpp	92.60% <ø> (ø)
src/solvers/prop/bdd_expr.cpp	93.02% <ø> (ø)
src/solvers/bdd/bdd_miniBDD.h	92.00% <0.00%> (-0.16%)	⬇️

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