Suppress warnings caused by use of -Wswitch_default on GCC

[jzmaddock]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 97.36842% with 2 lines in your changes missing coverage. Please review.

Project coverage is 94.1%. Comparing base (abc70a2) to head (bd18c8d).
Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/multiprecision/cpp_bin_float/io.hpp	0.0%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #651     +/-   ##
=========================================
+ Coverage     94.1%   94.1%   +0.1%     
=========================================
  Files          279     279             
  Lines        28903   28979     +76     
=========================================
+ Hits         27181   27255     +74     
- Misses        1722    1724      +2     

Files with missing lines	Coverage Δ
include/boost/multiprecision/cpp_bin_float.hpp	89.7% <100.0%> (+0.5%)	⬆️
...nclude/boost/multiprecision/detail/default_ops.hpp	91.1% <100.0%> (+0.2%)	⬆️
...lude/boost/multiprecision/detail/functions/pow.hpp	93.0% <100.0%> (+0.1%)	⬆️
...ude/boost/multiprecision/detail/functions/trig.hpp	96.4% <100.0%> (+0.1%)	⬆️
include/boost/multiprecision/cpp_bin_float/io.hpp	86.0% <0.0%> (-0.5%)	⬇️

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