fix MSVC most vexing parse

[sbiscigl]

Description of changes:

Currently CI is failing mainline on msvc with

D:\a\work\aws-crt-cpp\include\aws\crt\StringView.h(861,51): error C2760: syntax error: '>' was unexpected here; expected 'declaration' [D:\a\work\aws-crt-cpp\build\aws-crt-cpp\aws-crt-cpp-header-check-cxx20.vcxproj]
  (compiling source file '../../header-checker-cxx20/headerchecker_aws_crt_Api_h.cpp')

fixes most vexing parse by adding braced init + include string directly.

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