Resolve an ambiguous resolution of an overloaded function in G++ 6

[martin-cs]

Full details reported as:
#6258

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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).
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[martin-cs]

Thanks @peterschrammel for the swift review.

[codecov]

Codecov Report

Merging #6259 (3a2de48) into develop (2c32f86) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6259   +/-   ##
========================================
  Coverage    76.16%   76.16%           
========================================
  Files         1484     1484           
  Lines       162164   162164           
========================================
  Hits        123516   123516           
  Misses       38648    38648           

Impacted Files	Coverage Δ
src/solvers/smt2/smt2_parser.cpp	77.90% <100.00%> (ø)

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