[Windows][ros2] Avoid build break for Visual Studio 2019 v16.3

[seanyen]

From Visual Studio 2019 v16.3, the usage of the deprecated <experimental/filesystem> is escalated to be a compiler error like below:

C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.23.28105\include\experimental/filesystem(26,1): fatal error C1189: #error:  The <experimental/filesystem> header providing std::experimental::filesystem is deprecated by Microsoft and will be REMOVED. It is superseded by the C++17 <filesystem> header providing std::filesystem. You can define _SILENCE_EXPERIMENTAL_FILESYSTEM_DEPRECATION_WARNING to acknowledge that you have received this warning.

There are two ways to do about it:

1. Since <filesystem> is supported in Visual Studio 2017 v15.7, we can switch to use if __has_include(<filesystem>) && __cplusplus >= 201703L case, however, <filesystem> is considered a C++17 feature, so the whole project needs to target CMAKE_CXX_STANDARD 17 in order to make MSVC enable that, but which seems not to be the target platform for ROS2 Dashing. Hence, I didn't go with that route.

2. Define _SILENCE_EXPERIMENTAL_FILESYSTEM_DEPRECATION_WARNING to acknowledge that for now so it won't break the build. This one looks more conservative but it accommodates the current C++14 configuration.

[seanyen]

This is the same fix applied to pluginlib: ros/pluginlib#164

[seanyen]

cc @mjcarroll