[PowerPC] Fix a warning

This patch fixes:

  llvm/lib/Target/PowerPC/PPCSelectionDAGInfo.h:25:3: error:
  'EmitTargetCodeForMemcmp' overrides a member function but is not
  marked 'override' [-Werror,-Winconsistent-missing-override]

Author: kazutakahirata

llvm/lib/Target/PowerPC/PPCSelectionDAGInfo.h

@@ -24,7 +24,7 @@ class PPCSelectionDAGInfo : public SelectionDAGTargetInfo {
   std::pair<SDValue, SDValue>
   EmitTargetCodeForMemcmp(SelectionDAG &DAG, const SDLoc &dl, SDValue Chain,
                           SDValue Op1, SDValue Op2, SDValue Op3,
-                          const CallInst *CI) const;
+                          const CallInst *CI) const override;
 };
 
 } // namespace llvm