Fix file encoding of bigint-func.cc

[tautschnig]

Switch the file encoding of bigint-func.cc from ISO-8859 text to UTF-8 Unicode text. Inspection using file suggests this is the only source file with wrong encoding.

Fixes: #7509

• 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

Base: 78.16% // Head: 78.49% // Increases project coverage by +0.33% 🎉

Coverage data is based on head (1638191) compared to base (9b681ad).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7510      +/-   ##
===========================================
+ Coverage    78.16%   78.49%   +0.33%     
===========================================
  Files         1663     1663              
  Lines       191297   191297              
===========================================
+ Hits        149518   150154     +636     
+ Misses       41779    41143     -636     

Impacted Files	Coverage Δ
src/big-int/bigint-func.cc	33.89% <ø> (ø)
src/util/bitvector_expr.h	97.41% <0.00%> (+0.23%)	⬆️
src/util/std_expr.h	93.15% <0.00%> (+0.26%)	⬆️
src/util/std_code.cpp	41.89% <0.00%> (+1.35%)	⬆️
src/util/expr.h	97.14% <0.00%> (+1.90%)	⬆️
src/util/dstring.h	100.00% <0.00%> (+3.22%)	⬆️
src/util/symbol_table_base.cpp	67.85% <0.00%> (+3.57%)	⬆️
src/util/source_location.cpp	94.00% <0.00%> (+6.00%)	⬆️
src/util/simplify_utils.cpp	93.67% <0.00%> (+6.75%)	⬆️
src/util/union_find.cpp	82.81% <0.00%> (+7.81%)	⬆️
... and 25 more

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