[C++][Pistache] fix float literals in C++ Pistache codegen

[JulianGmp]

Small fix that ensures the float literals (written for default values) in the generated C++ code are suffixed with an f.
Before, this could cause a compile break because the literal would be interpreted as for example an integer before.
Note: this might be different for certain C++ compilers, I've encountered this with GCC 10.2.1.

It's a bit of a hack but hey it works.

Also: I ran the generate-samples script and did not encounter any changed files, so maybe this is an edge case not caught by the examples.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
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  Cc @ravinikam (2017/07) @stkrwork (2017/07) @etherealjoy (2018/02) @MartinDelille (2018/03) @muttleyxd (2019/08)