Which target SDK have you got setup in Projucer? 10.0.16299.0 is the recommended one as seen in this screenshot
Have you got 10.0.16299.0 installed? When you right click on #include wininet.h line in juce_BasicNativeHeaders.h what is the search path VS2017 reports? Is it looking in 10.0.16299.0 subdirectory?
Lastly, are you trying to build Projucer or a demo app or your own app? If you are building Projucer and you get missing wininet.h error, then try opening Projucer_App.vcxproj and replacing all 10.0.15063.0 with 10.0.16299.0 (after you get 10.0.16299.0 SDK).
I have to admit that I’ve had this problem as well, but it went away after a re-install of the Windows SDK via the VS installer. Maybe there was a broken update of VS which by mistake didn’t include this header?!? Anybody have any ideas?