Fix folder creation on Android

[jloehr]

Description

root doesn't has a trailing slash, so on Android a folder named filesoutput/ was created instead of files/output/.
This PR adds the missing directory separator when creating folders.

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[asuessenbach]

Under what circumstances do you get root without a trailing slash?
Maybe this function should be able handle both cases, root with and without a trailing slash.

[jloehr]

Under Android fs->external_storage_directory().string() (which is passed into create_path as root) returns something along /data/data/com.khronos.vulkan_samples/files (don't have the exact path at hand now) without the trailing space.

Vulkan-Samples/components/filesystem/src/legacy.cpp

Line 74 in 4c911f7

create_path(fs->external_storage_directory().string(), it->second);

seems to be the only place where create_path is called. We ran samples with that change on Android, Windows and Linux and didn't encounter any issues.

Maybe we can replace it with std::filesystem::create_directories altogether. Though haven't tested that. We ran with the simple added slash.