[NFC] Add InitializePlatformCommonFlags for Fuchsia

[leonardchan]

Summary

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.

[vitalybuka]

Sorry, feel free to land fixes like this without review.

[leonardchan]

In D90864#2376653, @vitalybuka wrote:

Sorry, feel free to land fixes like this without review.

No worries. Thanks!

compiler-rt/lib/sanitizer_common/sanitizer_fuchsia.cpp

 }
 
 u32 GetNumberOfCPUs() {
   return zx_system_get_num_cpus();
 }
 
 uptr GetRSS() { UNIMPLEMENTED(); }
 
+void InitializePlatformCommonFlags(CommonFlags *cf) {}
+
 }  // namespace __sanitizer
 
 using namespace __sanitizer;
 
 extern "C" {
 void __sanitizer_startup_hook(int argc, char **argv, char **envp,
                               void *stack_base, size_t stack_size) {
   __sanitizer::StoredArgv = argv;