[NFC] Add InitializePlatformCommonFlags for Fuchsia

https://reviews.llvm.org/D90811 is breaking our CI builders because
InitializePlatformCommonFlags is not defined. This just adds an empty definition.

This would've been caught on our upstream buildbot, but it's red at the moment
and most likely won't be sending out alert emails for recent failures.

Differential Revision: https://reviews.llvm.org/D90864