cp /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg ~/Desktop && open ~/Desktop/macOS_SDK_headers_for_macOS_10.14.pkg

Boom.  Now start removing all those pesky overrides you set in ~/.bundle/config

All credit goes to the amazing João Rocha da Silva.  Be sure to check out his detailed write-up on why you need to do this.  TLDR; Mac OS X 10.14 no longer places the include libraries in the standard /usr/include directory and locked by SIP.