Add a missing override keyword. NFC.

clang/lib/Basic/Targets/X86.h:293:8: warning: 'shouldEmitFloat16WithExcessPrecision' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
  bool shouldEmitFloat16WithExcessPrecision() const {
       ^
clang/include/clang/Basic/TargetInfo.h:915:16: note: overridden virtual function is here
  virtual bool shouldEmitFloat16WithExcessPrecision() const { return false; }
               ^