Enabling [-Werror,-Wunused-property-ivar] (#28895)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/28895

## Summary
Enabling [-Werror,-Wunused-property-ivar]

## Changelog
[Warning] [-Werror,-Wunused-property-ivar] Enable the warning

## Test Plan
Build

Reviewed By: jdthomas

Differential Revision: D20961613

fbshipit-source-id: 8ec73935384800581a71ad96957b716a0d894152

Libraries/Text/TextInput/RCTBaseTextInputView.m

@@ -304,6 +304,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)initWithFrame:(CGRect)frame)
 
 - (void)setShowSoftInputOnFocus:(BOOL)showSoftInputOnFocus
 {
+  (void)_showSoftInputOnFocus;
   if (showSoftInputOnFocus) {
     // Resets to default keyboard.
     self.backedTextInputView.inputView = nil;

React/CoreModules/BUCK

@@ -17,6 +17,7 @@ rn_apple_library(
     compiler_flags = [
         "-Wno-error=unguarded-availability-new",
         "-Wno-unknown-warning-option",
+        "-Wno-unused-property-ivar",
     ],
     contacts = ["oncall+react_native@xmail.facebook.com"],
     enable_exceptions = True,