[BUILD] Add missing includes to runtime_context_test

[perhapsmaple]

Fixes a build error when building opentelemetry-cpp with the following config:

cmake .. -DWITH_STL=ON -DCMAKE_CXX_STANDARD=20

Building WITH_STL=ON in Windows x64 with Visual Studio 2022 causes the following errors:

D:\repos\test\opentelemetry-cpp\api\test\context\runtime_context_test.cc(134,17): error C2039: 'next_permutation': is not a member of 'std' [D:\repos\test\opentelemetry-cpp\build\api\test\context\runtime_cont
ext_test.vcxproj]
D:\vcpkg\installed\x64-windows\include\gtest/internal/gtest-internal.h(1324,11): message : see declaration of 'std' [D:\repos\test\opentelemetry-cpp\build\api\test\context\runtime_context_test.vcxproj]
D:\repos\test\opentelemetry-cpp\api\test\context\runtime_context_test.cc(134,33): error C3861: 'next_permutation': identifier not found [D:\repos\test\opentelemetry-cpp\build\api\test\context\runtime_context_
test.vcxproj]

Building in linux with the same configuration using gcc 11.4.0 or clang-14 does not produce any errors, so the issue must be specific to MSVC.

Thanks to @Dangeroustuber for pointing out this issue.

Changes

Added the missing includes to the runtime_context_test.cc file.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[marcalff]

LGTM, thanks for the fix