/usr/include missing on macOS Catalina (with Xcode 11)
Set the CPATH environment variable in your shell (e.g., put this in your .zshrc assuming you're using zsh):

export CPATH=`xcrun --show-sdk-path`/usr/include


And then try to build your project again.

UPDATE

The OP related to how to install the headers in /usr/include, not how to point clang to the headers wherever they may be.

MacOS System Integrity Protection prevents the creation of /usr/include, even by root. See this related answer.

If you disabled SIP (not recommended) then I imagine something like

sudo ln -s `xcrun --show-sdk-path`/usr/include /usr/include


will do the trick.